[1/9] dt-bindings: mxsfb: Add compatible for i.MX8MP

Add compatible string for i.MX8MP LCDIF variant. This is called LCDIFv3
and is completely different from the LCDIFv3 found in i.MX23 in that it
has a completely scrambled register layout compared to all previous LCDIF
variants. The new LCDIFv3 also supports 36bit address space. However,
except for the complete bit reshuffling, this is still LCDIF and it still
works like one.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexander Stein <alexander.stein@ew.tq-group.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Robby Cai <robby.cai@nxp.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Stefan Agner <stefan@agner.ch>
Cc: devicetree@vger.kernel.org
---
 Documentation/devicetree/bindings/display/fsl,lcdif.yaml | 1 +
 1 file changed, 1 insertion(+)

Hi Marek, hi Liu,

Am Dienstag, dem 01.03.2022 um 10:44 +0800 schrieb Liu Ying:
> On Mon, 2022-02-28 at 16:34 +0100, Marek Vasut wrote:
> > On 2/28/22 09:18, Liu Ying wrote:
> > 
> > Hi,
> 
> Hi,
> 
> > 
> > > > > On Mon, 2022-02-28 at 01:45 +0100, Marek Vasut wrote:
> > > > > > Add compatible string for i.MX8MP LCDIF variant. This is called LCDIFv3
> > > > > > and is completely different from the LCDIFv3 found in i.MX23 in that it
> > > > > 
> > > > > In i.MX23 reference manual, there is no LCDIFv3 found, but only LCDIF.
> > > > 
> > > > See i.MX23 HW_LCDIF_VERSION MAJOR=0x3 , that's LCDIF V3 . MX28 has LCDIF
> > > > V4 .
> > > 
> > > Ok, got it now. AFAIK, the SoC design team calls i.MX8MP display
> > > controller as 'LCDIFv3'. Those in other SoCs are called 'LCDIF'.  There
> > > is not even a register in i.MX8MP display controller to decribe the
> > > version.
> > 
> > We also don't have a version register on MX6SX and we call it LCDIF V6 
> > in the driver. The naming scheme is confusing.
> 
> It looks ok for the current mxsfb drm driver to use its own version
> tracking mechanism to distinguish kinda small difference across LCDIF
> variants.  However, LCDIFv3 in i.MX8mp is a totally different IP, which
> does not apply to the tracking mechanism.
> 
> > 
> > > > > > has a completely scrambled register layout compared to all previous LCDIF
> > > > > 
> > > > > It looks like no single register of i.MX8MP LCDIFv3 overlaps with
> > > > > registers in other i.MX2x/6x/7x/8x LCDIFs. The LCDIFv3 block diagram is
> > > > > totally different from the LCDIF block diagram, according to the SoC
> > > > > reference manuals. LCDIFv3 supports SHADOW_EN bit to update horizontal
> > > > > and vertical size of graphic, position of graphic on the panel, address
> > > > > of graphic in memory and color formats or color palettes, which is not
> > > > > supported by LCDIF and impacts display driver control mechanism
> > > > > considerably. LCDIF supports DOTCLK interface, MPU interface and VSYNC
> > > > > interface, while LCDIFv3 only supports parallel output as a counterpart
> > > > > of the DOTCLK interface.
> > > > > 
> > > > > Generally speaking, LCDIFv3 is just a new display IP which happens to
> > > > > have the word 'LCDIF' in its name.  Although both of LCDIFv3 and LCDIF
> > > > > are display controllers for scanning out frames onto display devices, I
> > > > > don't think they are in one family.
> > > > > 
> > > > > So, LCDIFv3 deserves a new separate dt-binding, IMO.
> > > > 
> > > > It seems to me a lot of those bits just map to their previous
> > > > equivalents in older LCDIF, others were dropped, so this is some sort of
> > > > new LCDIF mutation, is it not ?
> > > 
> > > I say 'LCDIFv3' and 'LCDIF' are totally two IPs, if I compare the names
> > > of registers and the names of register bits .
> > > 
> > > > I am aware NXP has a separate driver in its downstream, but I'm not
> > > > convinced the duplication of boilerplate code by introducing a separate
> > > > driver for what looks like another LCDIF variant is the right approach.
> > > 
> > > Hmmm, given the two IPs, I think there should be separate drivers.
> > >   With one single driver, there would be too many 'if/else' checks to
> > > separate the logics for the IPs, just like Patch 9/9 does.  The
> > > boilerplate code to do things like registering a drm device is
> > > acceptable, IMO.
> > > 
> > > Aside from that, with separate drivers, we don't have to test too many
> > > SoCs if we only want to touch either 'LCDIFv3' or 'LCDIF'.
> > 
> > But then, with two drivers, you also might miss fixes which get applied 
> > to one driver and not the other, eventually the two drivers will diverge 
> > and that's not good.
> 
> Given the two totally different IPs, I don't see bugs of IP control
> logics should be fixed for both drivers. Naturally, the two would
> diverge due to different HWs. Looking at Patch 9/9, it basically
> squashes code to control LCDIFv3 into the mxsfb drm driver with
> 'if/else' checks(barely no common control code), which is hard to
> maintain and not able to achieve good scalability for both 'LCDIFv3'
> and 'LCDIF'.

I tend to agree with Liu here. Writing a DRM driver isn't that much
boilerplate anymore with all the helpers we have available in the
framework today.

The IP is so different from the currently supported LCDIF controllers
that I think trying to support this one in the existing driver actually
increases the chances to break something when modifying the driver in
the future. Not everyone is able to test all LCDIF versions. My vote is
on having a separate driver for the i.MX8MP LCDIF.

Regards,
Lucas

On 3/1/22 11:04, Lucas Stach wrote:

Hi,

[...]

>> Given the two totally different IPs, I don't see bugs of IP control
>> logics should be fixed for both drivers. Naturally, the two would
>> diverge due to different HWs. Looking at Patch 9/9, it basically
>> squashes code to control LCDIFv3 into the mxsfb drm driver with
>> 'if/else' checks(barely no common control code), which is hard to
>> maintain and not able to achieve good scalability for both 'LCDIFv3'
>> and 'LCDIF'.
> 
> I tend to agree with Liu here. Writing a DRM driver isn't that much
> boilerplate anymore with all the helpers we have available in the
> framework today.

I did write a separate driver for this IP before I spent time merging 
them into single driver, that's when I realized a single driver is much 
better and discarded the separate driver idea.

> The IP is so different from the currently supported LCDIF controllers
> that I think trying to support this one in the existing driver actually
> increases the chances to break something when modifying the driver in
> the future. Not everyone is able to test all LCDIF versions. My vote is
> on having a separate driver for the i.MX8MP LCDIF.

If you look at both controllers, it is clear it is still the LCDIF 
behind, even the CSC that is bolted on would suggest that.

I am also not happy when I look at the amount of duplication a separate 
driver would create, it will be some 50% of the code that would be just 
duplicated.

[...]

On Tue, Mar 1, 2022 at 5:05 AM Lucas Stach <l.stach@pengutronix.de> wrote:
>
> Am Dienstag, dem 01.03.2022 um 11:19 +0100 schrieb Marek Vasut:
> > On 3/1/22 11:04, Lucas Stach wrote:
> >
> > Hi,
> >
> > [...]
> >
> > > > Given the two totally different IPs, I don't see bugs of IP control
> > > > logics should be fixed for both drivers. Naturally, the two would
> > > > diverge due to different HWs. Looking at Patch 9/9, it basically
> > > > squashes code to control LCDIFv3 into the mxsfb drm driver with
> > > > 'if/else' checks(barely no common control code), which is hard to
> > > > maintain and not able to achieve good scalability for both 'LCDIFv3'
> > > > and 'LCDIF'.
> > >
> > > I tend to agree with Liu here. Writing a DRM driver isn't that much
> > > boilerplate anymore with all the helpers we have available in the
> > > framework today.
> >
> > I did write a separate driver for this IP before I spent time merging
> > them into single driver, that's when I realized a single driver is much
> > better and discarded the separate driver idea.
> >
> > > The IP is so different from the currently supported LCDIF controllers
> > > that I think trying to support this one in the existing driver actually
> > > increases the chances to break something when modifying the driver in
> > > the future. Not everyone is able to test all LCDIF versions. My vote is
> > > on having a separate driver for the i.MX8MP LCDIF.
> >
> > If you look at both controllers, it is clear it is still the LCDIF
> > behind, even the CSC that is bolted on would suggest that.
>
> Yes, but from a driver PoV what you care about is not really the
> hardware blocks used to implement something, but the programming model,
> i.e. the register interface exposed to software.
>
> >
> > I am also not happy when I look at the amount of duplication a separate
> > driver would create, it will be some 50% of the code that would be just
> > duplicated.
> >
> Yea, the duplicated code is still significant, as the HW itself is so
> simple. However, if you find yourself in the situation where basically
> every actual register access in the driver ends up being in a "if (some
> HW rev) ... " clause, i still think it would be better to have a
> separate driver, as the programming interface is just different.

I tend to agree with Marek on this one.  We have an instance where the
blk-ctrl and the GPC driver between 8m, mini, nano, plus are close,
but different enough where each SoC has it's own set of tables and
some checks.   Lucas created the framework, and others adapted it for
various SoC's.  If there really is nearly 50% common code for the
LCDIF, why not either leave the driver as one or split the common code
into its own driver like lcdif-common and then have smaller drivers
that handle their specific variations.

adam
>
> Regards,
> Lucas
>
>

>-----Original Message-----
>From: Lucas Stach <l.stach@pengutronix.de>
>Sent: 2022年3月1日 21:19
>To: Adam Ford <aford173@gmail.com>
>Cc: Marek Vasut <marex@denx.de>; Ying Liu (OSS) <victor.liu@oss.nxp.com>;
>dri-devel <dri-devel@lists.freedesktop.org>; devicetree
><devicetree@vger.kernel.org>; Peng Fan <peng.fan@nxp.com>; Alexander Stein
><alexander.stein@ew.tq-group.com>; Rob Herring <robh+dt@kernel.org>;
>Laurent Pinchart <laurent.pinchart@ideasonboard.com>; Sam Ravnborg
><sam@ravnborg.org>; Robby Cai <robby.cai@nxp.com>
>Subject: [EXT] Re: [PATCH 1/9] dt-bindings: mxsfb: Add compatible for
>i.MX8MP
>
>Caution: EXT Email
>
>Am Dienstag, dem 01.03.2022 um 07:03 -0600 schrieb Adam Ford:
>> On Tue, Mar 1, 2022 at 5:05 AM Lucas Stach <l.stach@pengutronix.de>
>wrote:
>> >
>> > Am Dienstag, dem 01.03.2022 um 11:19 +0100 schrieb Marek Vasut:
>> > > On 3/1/22 11:04, Lucas Stach wrote:
>> > >
>> > > Hi,
>> > >
>> > > [...]
>> > >
>> > > > > Given the two totally different IPs, I don't see bugs of IP
>> > > > > control logics should be fixed for both drivers. Naturally,
>> > > > > the two would diverge due to different HWs. Looking at Patch
>> > > > > 9/9, it basically squashes code to control LCDIFv3 into the
>> > > > > mxsfb drm driver with 'if/else' checks(barely no common
>> > > > > control code), which is hard to maintain and not able to achieve good
>scalability for both 'LCDIFv3'
>> > > > > and 'LCDIF'.
>> > > >
>> > > > I tend to agree with Liu here. Writing a DRM driver isn't that
>> > > > much boilerplate anymore with all the helpers we have available
>> > > > in the framework today.
>> > >
>> > > I did write a separate driver for this IP before I spent time
>> > > merging them into single driver, that's when I realized a single
>> > > driver is much better and discarded the separate driver idea.
>> > >
>> > > > The IP is so different from the currently supported LCDIF
>> > > > controllers that I think trying to support this one in the
>> > > > existing driver actually increases the chances to break
>> > > > something when modifying the driver in the future. Not everyone
>> > > > is able to test all LCDIF versions. My vote is on having a separate driver
>for the i.MX8MP LCDIF.
>> > >
>> > > If you look at both controllers, it is clear it is still the LCDIF
>> > > behind, even the CSC that is bolted on would suggest that.
>> >
>> > Yes, but from a driver PoV what you care about is not really the
>> > hardware blocks used to implement something, but the programming
>> > model, i.e. the register interface exposed to software.
>> >
>> > >
>> > > I am also not happy when I look at the amount of duplication a
>> > > separate driver would create, it will be some 50% of the code that
>> > > would be just duplicated.
>> > >
>> > Yea, the duplicated code is still significant, as the HW itself is
>> > so simple. However, if you find yourself in the situation where
>> > basically every actual register access in the driver ends up being
>> > in a "if (some HW rev) ... " clause, i still think it would be
>> > better to have a separate driver, as the programming interface is just
>different.
>>
>> I tend to agree with Marek on this one.  We have an instance where the
>> blk-ctrl and the GPC driver between 8m, mini, nano, plus are close,
>> but different enough where each SoC has it's own set of tables and
>> some checks.   Lucas created the framework, and others adapted it for
>> various SoC's.  If there really is nearly 50% common code for the
>> LCDIF, why not either leave the driver as one or split the common code
>> into its own driver like lcdif-common and then have smaller drivers
>> that handle their specific variations.
>
>I don't know exactly how the standalone driver looks like, but I guess the
>overlap is not really in any real HW specific parts, but the common DRM
>boilerplate, so there isn't much point in creating a common lcdif driver.
>
>As you brought up the blk-ctrl as an example: I'm all for supporting slightly
>different hardware in the same driver, as long as the HW interface is close
>enough. But then I also opted for a separate 8MP blk-ctrl driver for those
>blk-ctrls that differ significantly from the others, as I think it would make the
>common driver unmaintainable trying to support all the different variants in
>one driver.
>
>Regards,
>Lucas

LCDIF on i.MX8MP is a different IP which is borrowed from non-iMX series, although it's also called 'LCDIF'.
We prefer not mix these two series of IPs in one driver for ease of maintenance and extension.

Regards,
Robby