| Message ID | 87ip2wr8tp.fsf@rustcorp.com.au |
|---|---|
| State | New |
| Headers | show |
On 6 May 2013 06:11, Rusty Russell <rusty@rustcorp.com.au> wrote: > Peter Maydell <peter.maydell@linaro.org> writes: >> To be actually useful we need to also specify something in >> the device tree to say "here is where you will find your >> emergency output and what it is". > > Hmm, I'm not sure that's true. It looks like it needs: > > 1) An enhancment to the vdev->set_config callback to pass through (at > least) an offset, probably offset and length. > > 2) An emerg_write() function ptr which can be called at any time, set by > virtio_console.c's class_init: That all looks like sensible QEMU implementation possibilities but it seems to be a bit of a non-sequitur from "how do we tell the kernel to actually use this?" -- PMM
Hi Rusty, On 6 May 2013 10:41, Rusty Russell <rusty@rustcorp.com.au> wrote: > Peter Maydell <peter.maydell@linaro.org> writes: >> On 1 May 2013 03:07, Rusty Russell <rusty@rustcorp.com.au> wrote: >>> An emergency output is a reasonable idea, and this is a reasonable >>> implementation. The question is practical: will it be used? Because we >>> don't implement reasonable ideas which aren't going to be used. >> >> If you think it fits reasonably into the virtio spec (ie doesn't >> implement things at the wrong level of the transport/backend >> abstraction) then we can implement it in QEMU, and I think it >> makes more sense to do this than to throw in a random extra >> serial port. >> >> To be actually useful we need to also specify something in >> the device tree to say "here is where you will find your >> emergency output and what it is". > > Hmm, I'm not sure that's true. It looks like it needs: > > 1) An enhancment to the vdev->set_config callback to pass through (at > least) an offset, probably offset and length. > > 2) An emerg_write() function ptr which can be called at any time, set by > virtio_console.c's class_init: > > static void emerg_write(VirtIOSerialPort *port, char c) > { > VirtConsole *vcon = DO_UPCAST(VirtConsole, port, port); > > if (vcon->chr) > qemu_chr_fe_write(vcon->chr, &c, 1); > } > > 3) A routine to find an emerg-write-capable console in > virtio_serial_bus.c (or just assume port 0?): > > static VirtIOSerialPort *find_emerg_write_port(VirtIOSerial *vser) > { > VirtIOSerialPort *port; > > QTAILQ_FOREACH(port, &vser->ports, next) { > VirtIOSerialPortClass *vsc; > vsc = VIRTIO_SERIAL_PORT_GET_CLASS(port); > > if (vsc->emerg_write) { > return port; > } > } > return NULL; > } > > 4) set_config in virtio_serial_bus.c to notice emergency writes: > > if (offset == offsetof(struct virtio_console_config, emerg_w) { > VirtIOSerial *vser; > vser = DO_UPCAST(VirtIOSerial, vdev, vdev); > VirtIOSerialPort *port; > > port = find_emerg_write_port(vser); > if (port) { > vsc->emerg_write(port, config.emerg_w); > } > } > > Amit might have more clue... Amit? > > Thanks, > Rusty. > > From: Pranavkumar Sawargaonkar <pranavkumar@linaro.org> > Subject: virtio: console: Add early writeonly register to config space > > This patch adds a emerg_wr register (writeonly) in config space of virtio console device which can be used for debugging. > > Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org> > Signed-off-by: Anup Patel <anup.patel@linaro.org> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> > --- > include/uapi/linux/virtio_console.h | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/include/uapi/linux/virtio_console.h b/include/uapi/linux/virtio_console.h > index ee13ab6..586678d 100644 > --- a/include/uapi/linux/virtio_console.h > +++ b/include/uapi/linux/virtio_console.h > @@ -38,6 +38,7 @@ > /* Feature bits */ > #define VIRTIO_CONSOLE_F_SIZE 0 /* Does host provide console size? */ > #define VIRTIO_CONSOLE_F_MULTIPORT 1 /* Does host provide multiple ports? */ > +#define VIRTIO_CONSOLE_F_EMERG_WRITE 2 /* Does host support emergency write? */ > > #define VIRTIO_CONSOLE_BAD_ID (~(u32)0) > > @@ -48,6 +49,8 @@ struct virtio_console_config { > __u16 rows; > /* max. number of ports this device can hold */ > __u32 max_nr_ports; > + /* emergency write register */ > + __u32 emerg_wr; > } __attribute__((packed)); > > /* I will send above patch with V2 tagged with the addition of documentation change and also separate arm64 early printk implementation for the same. Thanks, Pranav
Peter Maydell <peter.maydell@linaro.org> writes: > On 6 May 2013 06:11, Rusty Russell <rusty@rustcorp.com.au> wrote: >> Peter Maydell <peter.maydell@linaro.org> writes: >>> To be actually useful we need to also specify something in >>> the device tree to say "here is where you will find your >>> emergency output and what it is". >> >> Hmm, I'm not sure that's true. It looks like it needs: >> >> 1) An enhancment to the vdev->set_config callback to pass through (at >> least) an offset, probably offset and length. >> >> 2) An emerg_write() function ptr which can be called at any time, set by >> virtio_console.c's class_init: > > That all looks like sensible QEMU implementation possibilities > but it seems to be a bit of a non-sequitur from "how do we > tell the kernel to actually use this?" You enable the feature in the virtio console device, and a kernel compiled with EARLY_PRINTK will use it? Cheers, Rusty.
On 7 May 2013 05:46, Rusty Russell <rusty@rustcorp.com.au> wrote: > Peter Maydell <peter.maydell@linaro.org> writes: >> That all looks like sensible QEMU implementation possibilities >> but it seems to be a bit of a non-sequitur from "how do we >> tell the kernel to actually use this?" > > You enable the feature in the virtio console device, and a kernel > compiled with EARLY_PRINTK will use it? Well, at the moment EARLY_PRINTK is hardcoded to "use some specific UART or equivalent selected at compile time". So the equivalent presumably would be to hard-compile "use virtio-console", but then how does that code know where the virtio-console is in the address space? -- PMM
diff --git a/include/uapi/linux/virtio_console.h b/include/uapi/linux/virtio_console.h index ee13ab6..586678d 100644 --- a/include/uapi/linux/virtio_console.h +++ b/include/uapi/linux/virtio_console.h @@ -38,6 +38,7 @@ /* Feature bits */ #define VIRTIO_CONSOLE_F_SIZE 0 /* Does host provide console size? */ #define VIRTIO_CONSOLE_F_MULTIPORT 1 /* Does host provide multiple ports? */ +#define VIRTIO_CONSOLE_F_EMERG_WRITE 2 /* Does host support emergency write? */ #define VIRTIO_CONSOLE_BAD_ID (~(u32)0) @@ -48,6 +49,8 @@ struct virtio_console_config { __u16 rows; /* max. number of ports this device can hold */ __u32 max_nr_ports; + /* emergency write register */ + __u32 emerg_wr; } __attribute__((packed)); /*