| Message ID | 20240515-dma-buf-ecc-heap-v1-0-54cbbd049511@kernel.org |
|---|---|
| Headers | show |
| Series | dma-buf: heaps: Support carved-out heaps and ECC related-flags | expand |
On Wed, May 15, 2024 at 6:57 AM Maxime Ripard <mripard@kernel.org> wrote: > This series is the follow-up of the discussion that John and I had a few > months ago here: > > https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/ > > The initial problem we were discussing was that I'm currently working on > a platform which has a memory layout with ECC enabled. However, enabling > the ECC has a number of drawbacks on that platform: lower performance, > increased memory usage, etc. So for things like framebuffers, the > trade-off isn't great and thus there's a memory region with ECC disabled > to allocate from for such use cases. > > After a suggestion from John, I chose to start using heap allocations > flags to allow for userspace to ask for a particular ECC setup. This is > then backed by a new heap type that runs from reserved memory chunks > flagged as such, and the existing DT properties to specify the ECC > properties. > > We could also easily extend this mechanism to support more flags, or > through a new ioctl to discover which flags a given heap supports. Hey! Thanks for sending this along! I'm eager to see more heap related work being done upstream. The only thing that makes me a bit hesitant, is the introduction of allocation flags (as opposed to a uniquely specified/named "ecc" heap). We did talk about this earlier, and my earlier press that only if the ECC flag was general enough to apply to the majority of heaps then it makes sense as a flag, and your patch here does apply it to all the heaps. So I don't have an objection. But it makes me a little nervous to add a new generic allocation flag for a feature most hardware doesn't support (yet, at least). So it's hard to weigh how common the actual usage will be across all the heaps. I apologize as my worry is mostly born out of seeing vendors really push opaque feature flags in their old ion heaps, so in providing a flags argument, it was mostly intended as an escape hatch for obviously common attributes. So having the first be something that seems reasonable, but isn't actually that common makes me fret some. So again, not an objection, just something for folks to stew on to make sure this is really the right approach. Another thing to discuss, that I didn't see in your mail: Do we have an open-source user of this new flag? thanks -john
Am 15.05.24 um 15:56 schrieb Maxime Ripard: > The uAPI header has a bunch of constants and structures that might be > handy in drivers. > > Let's include the header in the driver-side dma-heap header. Well as long as this header doesn't need any symbols from the uAPI itself I think that is a no-go. Includes should only be applied for things which are really necessary and not because some driver might need it. Regards, Christian. > > Signed-off-by: Maxime Ripard <mripard@kernel.org> > --- > include/linux/dma-heap.h | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h > index 0c05561cad6e..e7cf110c5fdc 100644 > --- a/include/linux/dma-heap.h > +++ b/include/linux/dma-heap.h > @@ -10,10 +10,12 @@ > #define _DMA_HEAPS_H > > #include <linux/cdev.h> > #include <linux/types.h> > > +#include <uapi/linux/dma-heap.h> > + > struct dma_heap; > > /** > * struct dma_heap_ops - ops to operate on a given heap > * @allocate: allocate dmabuf and return struct dma_buf ptr >
On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > On Wed, May 15, 2024 at 6:57 AM Maxime Ripard <mripard@kernel.org> wrote: > > This series is the follow-up of the discussion that John and I had a few > > months ago here: > > > > https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/ > > > > The initial problem we were discussing was that I'm currently working on > > a platform which has a memory layout with ECC enabled. However, enabling > > the ECC has a number of drawbacks on that platform: lower performance, > > increased memory usage, etc. So for things like framebuffers, the > > trade-off isn't great and thus there's a memory region with ECC disabled > > to allocate from for such use cases. > > > > After a suggestion from John, I chose to start using heap allocations > > flags to allow for userspace to ask for a particular ECC setup. This is > > then backed by a new heap type that runs from reserved memory chunks > > flagged as such, and the existing DT properties to specify the ECC > > properties. > > > > We could also easily extend this mechanism to support more flags, or > > through a new ioctl to discover which flags a given heap supports. > > Hey! Thanks for sending this along! I'm eager to see more heap related > work being done upstream. > > The only thing that makes me a bit hesitant, is the introduction of > allocation flags (as opposed to a uniquely specified/named "ecc" > heap). > > We did talk about this earlier, and my earlier press that only if the > ECC flag was general enough to apply to the majority of heaps then it > makes sense as a flag, and your patch here does apply it to all the > heaps. So I don't have an objection. > > But it makes me a little nervous to add a new generic allocation flag > for a feature most hardware doesn't support (yet, at least). So it's > hard to weigh how common the actual usage will be across all the > heaps. > > I apologize as my worry is mostly born out of seeing vendors really > push opaque feature flags in their old ion heaps, so in providing a > flags argument, it was mostly intended as an escape hatch for > obviously common attributes. So having the first be something that > seems reasonable, but isn't actually that common makes me fret some. > > So again, not an objection, just something for folks to stew on to > make sure this is really the right approach. Another good reason to go with full heap names instead of opaque flags on existing heaps is that with the former we can use symlinks in sysfs to specify heaps, with the latter we need a new idea. We haven't yet gotten around to implement this anywhere, but it's been in the dma-buf/heap todo since forever, and I like it as a design approach. So would be a good idea to not toss it. With that display would have symlinks to cma-ecc and cma, and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a SoC where the display needs contig memory for scanout. > Another thing to discuss, that I didn't see in your mail: Do we have > an open-source user of this new flag? I think one option might be to just start using these internally, but not sure the dma-api would understand a fallback cadence of allocators (afaik you can specify specific cma regions already, but that doesn't really covere the case where you can fall back to pages and iommu to remap to contig dma space) ... And I don't think abandonding the dma-api for allocating cma buffers is going to be a popular proposal. -Sima
Hi John, Thanks for your feedback On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > On Wed, May 15, 2024 at 6:57 AM Maxime Ripard <mripard@kernel.org> wrote: > > This series is the follow-up of the discussion that John and I had a few > > months ago here: > > > > https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/ > > > > The initial problem we were discussing was that I'm currently working on > > a platform which has a memory layout with ECC enabled. However, enabling > > the ECC has a number of drawbacks on that platform: lower performance, > > increased memory usage, etc. So for things like framebuffers, the > > trade-off isn't great and thus there's a memory region with ECC disabled > > to allocate from for such use cases. > > > > After a suggestion from John, I chose to start using heap allocations > > flags to allow for userspace to ask for a particular ECC setup. This is > > then backed by a new heap type that runs from reserved memory chunks > > flagged as such, and the existing DT properties to specify the ECC > > properties. > > > > We could also easily extend this mechanism to support more flags, or > > through a new ioctl to discover which flags a given heap supports. > > Hey! Thanks for sending this along! I'm eager to see more heap related > work being done upstream. > > The only thing that makes me a bit hesitant, is the introduction of > allocation flags (as opposed to a uniquely specified/named "ecc" > heap). > > We did talk about this earlier, and my earlier press that only if the > ECC flag was general enough to apply to the majority of heaps then it > makes sense as a flag, and your patch here does apply it to all the > heaps. So I don't have an objection. > > But it makes me a little nervous to add a new generic allocation flag > for a feature most hardware doesn't support (yet, at least). So it's > hard to weigh how common the actual usage will be across all the > heaps. > > I apologize as my worry is mostly born out of seeing vendors really > push opaque feature flags in their old ion heaps, so in providing a > flags argument, it was mostly intended as an escape hatch for > obviously common attributes. So having the first be something that > seems reasonable, but isn't actually that common makes me fret some. > > So again, not an objection, just something for folks to stew on to > make sure this is really the right approach. I understand your hesitation and concern :) Is there anything we could provide that would help moving the discussion forward? > Another thing to discuss, that I didn't see in your mail: Do we have > an open-source user of this new flag? Not at the moment. I guess it would be one of the things that would reduce your concerns, but is it a requirement? Thanks! Maxime
On Thu, May 16, 2024 at 12:56:27PM +0200, Daniel Vetter wrote: > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > > On Wed, May 15, 2024 at 6:57 AM Maxime Ripard <mripard@kernel.org> wrote: > > > This series is the follow-up of the discussion that John and I had a few > > > months ago here: > > > > > > https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/ > > > > > > The initial problem we were discussing was that I'm currently working on > > > a platform which has a memory layout with ECC enabled. However, enabling > > > the ECC has a number of drawbacks on that platform: lower performance, > > > increased memory usage, etc. So for things like framebuffers, the > > > trade-off isn't great and thus there's a memory region with ECC disabled > > > to allocate from for such use cases. > > > > > > After a suggestion from John, I chose to start using heap allocations > > > flags to allow for userspace to ask for a particular ECC setup. This is > > > then backed by a new heap type that runs from reserved memory chunks > > > flagged as such, and the existing DT properties to specify the ECC > > > properties. > > > > > > We could also easily extend this mechanism to support more flags, or > > > through a new ioctl to discover which flags a given heap supports. > > > > Hey! Thanks for sending this along! I'm eager to see more heap related > > work being done upstream. > > > > The only thing that makes me a bit hesitant, is the introduction of > > allocation flags (as opposed to a uniquely specified/named "ecc" > > heap). > > > > We did talk about this earlier, and my earlier press that only if the > > ECC flag was general enough to apply to the majority of heaps then it > > makes sense as a flag, and your patch here does apply it to all the > > heaps. So I don't have an objection. > > > > But it makes me a little nervous to add a new generic allocation flag > > for a feature most hardware doesn't support (yet, at least). So it's > > hard to weigh how common the actual usage will be across all the > > heaps. > > > > I apologize as my worry is mostly born out of seeing vendors really > > push opaque feature flags in their old ion heaps, so in providing a > > flags argument, it was mostly intended as an escape hatch for > > obviously common attributes. So having the first be something that > > seems reasonable, but isn't actually that common makes me fret some. > > > > So again, not an objection, just something for folks to stew on to > > make sure this is really the right approach. > > Another good reason to go with full heap names instead of opaque flags on > existing heaps is that with the former we can use symlinks in sysfs to > specify heaps, with the latter we need a new idea. We haven't yet gotten > around to implement this anywhere, but it's been in the dma-buf/heap todo > since forever, and I like it as a design approach. So would be a good idea > to not toss it. With that display would have symlinks to cma-ecc and cma, > and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a > SoC where the display needs contig memory for scanout. I guess it depends what we want to use the heaps for exactly. If we create a heap by type, then the number of heaps is going to explode and their name is going to be super weird and inconsistent. Using the ECC setup here as an example, it means that we would need to create system (with the default ECC setup for the system), system-ecc, system-no-ecc, cma, cma-ecc, cma-no-ecc. Let's say we introduce caching next. do we want to triple the number of heaps again? So I guess it all boils down to whether we want to consider heaps as allocators, and then we need the flags to fine-tune the attributes/exact semantics, or the combination of an allocator and the semantics which will make the number of heaps explode (and reduce their general usefulness, I guess). Maxime
On Thu, May 16, 2024 at 3:56 AM Daniel Vetter <daniel@ffwll.ch> wrote: > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > > But it makes me a little nervous to add a new generic allocation flag > > for a feature most hardware doesn't support (yet, at least). So it's > > hard to weigh how common the actual usage will be across all the > > heaps. > > > > I apologize as my worry is mostly born out of seeing vendors really > > push opaque feature flags in their old ion heaps, so in providing a > > flags argument, it was mostly intended as an escape hatch for > > obviously common attributes. So having the first be something that > > seems reasonable, but isn't actually that common makes me fret some. > > > > So again, not an objection, just something for folks to stew on to > > make sure this is really the right approach. > > Another good reason to go with full heap names instead of opaque flags on > existing heaps is that with the former we can use symlinks in sysfs to > specify heaps, with the latter we need a new idea. We haven't yet gotten > around to implement this anywhere, but it's been in the dma-buf/heap todo > since forever, and I like it as a design approach. So would be a good idea > to not toss it. With that display would have symlinks to cma-ecc and cma, > and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a > SoC where the display needs contig memory for scanout. So indeed that is a good point to keep in mind, but I also think it might re-inforce the choice of having ECC as a flag here. Since my understanding of the sysfs symlinks to heaps idea is about being able to figure out a common heap from a collection of devices, it's really about the ability for the driver to access the type of memory. If ECC is just an attribute of the type of memory (as in this patch series), it being on or off won't necessarily affect compatibility of the buffer with the device. Similarly "uncached" seems more of an attribute of memory type and not a type itself. Hardware that can access non-contiguous "system" buffers can access uncached system buffers. thanks -john
On Thu, May 16, 2024 at 5:22 AM Maxime Ripard <mripard@kernel.org> wrote: > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > > I apologize as my worry is mostly born out of seeing vendors really > > push opaque feature flags in their old ion heaps, so in providing a > > flags argument, it was mostly intended as an escape hatch for > > obviously common attributes. So having the first be something that > > seems reasonable, but isn't actually that common makes me fret some. > > > > So again, not an objection, just something for folks to stew on to > > make sure this is really the right approach. > > I understand your hesitation and concern :) Is there anything we could > provide that would help moving the discussion forward? > Mostly I just want to make sure it's discussed, which is why I raise it as a concern. Getting APIs right is hard, and I know I've made my share of mistakes (for instance, a situation that very much echoes this current question: the *_ALARM clockids probably should have used a flag). So I've found highlighting the trade-offs and getting other folk's perspectives useful to avoid such issues. But I also don't intend to needlessly delay things. > > Another thing to discuss, that I didn't see in your mail: Do we have > > an open-source user of this new flag? > > Not at the moment. I guess it would be one of the things that would > reduce your concerns, but is it a requirement? So I'd defer to Sima on this. There have been a number of heap releated changes that have had to be held out of tree on this requirement, but I'm hoping recent efforts on upstream device support will eventually unblock those. thanks -john
On Thu, May 16, 2024 at 09:51:35AM -0700, John Stultz wrote: > On Thu, May 16, 2024 at 3:56 AM Daniel Vetter <daniel@ffwll.ch> wrote: > > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > > > But it makes me a little nervous to add a new generic allocation flag > > > for a feature most hardware doesn't support (yet, at least). So it's > > > hard to weigh how common the actual usage will be across all the > > > heaps. > > > > > > I apologize as my worry is mostly born out of seeing vendors really > > > push opaque feature flags in their old ion heaps, so in providing a > > > flags argument, it was mostly intended as an escape hatch for > > > obviously common attributes. So having the first be something that > > > seems reasonable, but isn't actually that common makes me fret some. > > > > > > So again, not an objection, just something for folks to stew on to > > > make sure this is really the right approach. > > > > Another good reason to go with full heap names instead of opaque flags on > > existing heaps is that with the former we can use symlinks in sysfs to > > specify heaps, with the latter we need a new idea. We haven't yet gotten > > around to implement this anywhere, but it's been in the dma-buf/heap todo > > since forever, and I like it as a design approach. So would be a good idea > > to not toss it. With that display would have symlinks to cma-ecc and cma, > > and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a > > SoC where the display needs contig memory for scanout. > > So indeed that is a good point to keep in mind, but I also think it > might re-inforce the choice of having ECC as a flag here. > > Since my understanding of the sysfs symlinks to heaps idea is about > being able to figure out a common heap from a collection of devices, > it's really about the ability for the driver to access the type of > memory. If ECC is just an attribute of the type of memory (as in this > patch series), it being on or off won't necessarily affect > compatibility of the buffer with the device. Similarly "uncached" > seems more of an attribute of memory type and not a type itself. > Hardware that can access non-contiguous "system" buffers can access > uncached system buffers. Yeah, but in graphics there's a wide band where "shit performance" is defacto "not useable (as intended at least)". So if we limit the symlink idea to just making sure zero-copy access is possible, then we might not actually solve the real world problem we need to solve. And so the symlinks become somewhat useless, and we need to somewhere encode which flags you need to use with each symlink. But I also see the argument that there's a bit a combinatorial explosion possible. So I guess the question is where we want to handle it ... Also wondering whether we should get the symlink/allocator idea off the ground first, but given that that hasn't moved in a decade it might be too much. But then the question is, what userspace are we going to use for all these new heaps (or heaps with new flags)? Cheers, Sima
On Tue, May 21, 2024 at 02:06:19PM GMT, Daniel Vetter wrote: > On Thu, May 16, 2024 at 09:51:35AM -0700, John Stultz wrote: > > On Thu, May 16, 2024 at 3:56 AM Daniel Vetter <daniel@ffwll.ch> wrote: > > > On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote: > > > > But it makes me a little nervous to add a new generic allocation flag > > > > for a feature most hardware doesn't support (yet, at least). So it's > > > > hard to weigh how common the actual usage will be across all the > > > > heaps. > > > > > > > > I apologize as my worry is mostly born out of seeing vendors really > > > > push opaque feature flags in their old ion heaps, so in providing a > > > > flags argument, it was mostly intended as an escape hatch for > > > > obviously common attributes. So having the first be something that > > > > seems reasonable, but isn't actually that common makes me fret some. > > > > > > > > So again, not an objection, just something for folks to stew on to > > > > make sure this is really the right approach. > > > > > > Another good reason to go with full heap names instead of opaque flags on > > > existing heaps is that with the former we can use symlinks in sysfs to > > > specify heaps, with the latter we need a new idea. We haven't yet gotten > > > around to implement this anywhere, but it's been in the dma-buf/heap todo > > > since forever, and I like it as a design approach. So would be a good idea > > > to not toss it. With that display would have symlinks to cma-ecc and cma, > > > and rendering maybe cma-ecc, shmem, cma heaps (in priority order) for a > > > SoC where the display needs contig memory for scanout. > > > > So indeed that is a good point to keep in mind, but I also think it > > might re-inforce the choice of having ECC as a flag here. > > > > Since my understanding of the sysfs symlinks to heaps idea is about > > being able to figure out a common heap from a collection of devices, > > it's really about the ability for the driver to access the type of > > memory. If ECC is just an attribute of the type of memory (as in this > > patch series), it being on or off won't necessarily affect > > compatibility of the buffer with the device. Similarly "uncached" > > seems more of an attribute of memory type and not a type itself. > > Hardware that can access non-contiguous "system" buffers can access > > uncached system buffers. > > Yeah, but in graphics there's a wide band where "shit performance" is > defacto "not useable (as intended at least)". Right, but "not useable" is still kind of usage dependent, which reinforces the need for flags (and possibly some way to discover what the heap supports). Like, if I just want to allocate a buffer for a single writeback frame, then I probably don't have the same requirements than a compositor that needs to output a frame at 120Hz. The former probably doesn't care about the buffer attributes aside that it's accessible by the device. The latter probably can't make any kind of compromise over what kind of memory characteristics it uses. If we look into the current discussions we have, a compositor would probably need a buffer without ECC, non-secure, and probably wouldn't care about caching and being physically contiguous. Libcamera's SoftISP would probably require that the buffer is cacheable, non-secure, without ECC and might ask for physically contiguous buffers. As we add more memory types / attributes, I think being able to discover and enforce a particular set of flags will be more and more important, even more so if we tie heaps to devices, because it just gives a hint about the memory being reachable from the device, but as you said, you can still get a buffer with shit performance that won't be what you want. > So if we limit the symlink idea to just making sure zero-copy access is > possible, then we might not actually solve the real world problem we need > to solve. And so the symlinks become somewhat useless, and we need to > somewhere encode which flags you need to use with each symlink. > > But I also see the argument that there's a bit a combinatorial explosion > possible. So I guess the question is where we want to handle it ... > > Also wondering whether we should get the symlink/allocator idea off the > ground first, but given that that hasn't moved in a decade it might be too > much. But then the question is, what userspace are we going to use for all > these new heaps (or heaps with new flags)? For ECC here, the compositors are the obvious target. Which loops backs into the discussion with John. Do you consider dma-buf code have the same uapi requirements as DRM? Maxime
Hi, This series is the follow-up of the discussion that John and I had a few months ago here: https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/ The initial problem we were discussing was that I'm currently working on a platform which has a memory layout with ECC enabled. However, enabling the ECC has a number of drawbacks on that platform: lower performance, increased memory usage, etc. So for things like framebuffers, the trade-off isn't great and thus there's a memory region with ECC disabled to allocate from for such use cases. After a suggestion from John, I chose to start using heap allocations flags to allow for userspace to ask for a particular ECC setup. This is then backed by a new heap type that runs from reserved memory chunks flagged as such, and the existing DT properties to specify the ECC properties. We could also easily extend this mechanism to support more flags, or through a new ioctl to discover which flags a given heap supports. I submitted a draft PR to the DT schema for the bindings used in this PR: https://github.com/devicetree-org/dt-schema/pull/138 Let me know what you think, Maxime Signed-off-by: Maxime Ripard <mripard@kernel.org> --- Maxime Ripard (8): dma-buf: heaps: Introduce a new heap for reserved memory of: Add helper to retrieve ECC memory bits dma-buf: heaps: Import uAPI header dma-buf: heaps: Add ECC protection flags dma-buf: heaps: system: Remove global variable dma-buf: heaps: system: Handle ECC flags dma-buf: heaps: cma: Handle ECC flags dma-buf: heaps: carveout: Handle ECC flags drivers/dma-buf/dma-heap.c | 4 + drivers/dma-buf/heaps/Kconfig | 8 + drivers/dma-buf/heaps/Makefile | 1 + drivers/dma-buf/heaps/carveout_heap.c | 330 ++++++++++++++++++++++++++++++++++ drivers/dma-buf/heaps/cma_heap.c | 10 ++ drivers/dma-buf/heaps/system_heap.c | 29 ++- include/linux/dma-heap.h | 2 + include/linux/of.h | 25 +++ include/uapi/linux/dma-heap.h | 5 +- 9 files changed, 407 insertions(+), 7 deletions(-) --- base-commit: a38297e3fb012ddfa7ce0321a7e5a8daeb1872b6 change-id: 20240515-dma-buf-ecc-heap-28a311d2c94e Best regards,