[PATCH v3] drm/plane: Add documentation about software color conversion.

Fri Sep 8 13:56:51 UTC 2023

Hi

Am 08.09.23 um 13:16 schrieb Pekka Paalanen:
> On Fri, 8 Sep 2023 11:21:51 +0200
> Thomas Zimmermann <tzimmermann at suse.de> wrote:
> 
>> Hi
>>
>> Am 25.08.23 um 16:04 schrieb Jocelyn Falempe:
>> [...]
>>> + *
>>> + *     But there are two exceptions only for dumb buffers:
>>> + *     * To support XRGB8888 if it's not supported by the hardware.
>>
>>
>>> + *     * Any driver is free to modify its internal representation of the format,
>>> + *       as long as it doesn't alter the visible content in any way, and doesn't
>>> + *       modify the user-provided buffer. An example would be to drop the
>>> + *       padding component from a format to save some memory bandwidth.
>>
>> I have strong objections to this point, _especially_ as you're
>> apparently trying to sneak this in after our discussion. NAK on this
>> part from my side.
>>
>> If you want userspace to be able to use a certain format, then export
>> the corresponding 4cc code. Then let userspace decide what to do about
>> it. If userspace pick a certain format, go with it.
> 
> What is the reason for your objection, exactly?
> 
>> Hence, no implicit conversion from XRGB888 to RGB888, just because it's
>> possible.
> 
> For the particular driver in question though, the conversion allows
> using a display resolution that is otherwise not possible. I also hear
> it improves performance since 25% less data needs to travel across a
> slow bus. There is also so little VRAM, than all dumb buffers need to
> be allocated from sysram instead anyway, so a copy is always necessary.
> 
> Since XRGB8888 is the one format that is recommended to be supported by
> all drivers, I don't see a problem here. Did you test on your
> incredibly slow g200 test rig if the conversion ends up hurting instead
> of helping performance there?
> 
> If it hurts, then I see that you have a good reason to NAK this.
> 
> It's hard to imagine how it would hurt, since you always need a copy
> from sysram dumb buffers to VRAM - or do you?

I have a number of concerns. My point it not that we shouldn't optimize. 
I just don't want it in the kernel. Mgag200 can export DRM_FORMAT_RGB888 
for userspace to use.

AFAICT the main argument against userspace is that Mesa doesn't like 
3-byte pixels. But I don't see how this conversion cannot be a 
post-processing step within Mesa: do the rendering in RGB32 and then 
convert to a framebuffer in RGB24. Userspace can do that more 
efficiently than the kernel. This has all of the upsides of reduced 
bandwidth, but none of the downsides of kernel code. Applications and/or 
Mesa would be in control of the buffer format and apply the optimization 
where it makes sense. And it would be available for all drivers that are 
similar to mgag200.

My main point is simplicity of the driver: I prefer the driver to be 
simple without unnecessary indirection or overhead. Optimizations like 
these my or may not work on a given system with a certain workload. I'd 
better leave this heuristic to userspace.

Another point of concern is CPU consumption: Slow I/O buses may stall 
the display thread, but the CPU could do something else in the meantime. 
Doing format conversion on the CPU prevents that, hence affecting other 
parts of the system negatively. Of course, that's more of a gut feeling 
than hard data.

Please note that the kernel's conversion code uses memory allocation of 
intermediate buffers. We even recently had a discussion about allocation 
overhead during display updates. Userspace can surely do a better job at 
keeping such buffers around.

And finally a note the hardware itself: on low-end hardware like those 
Matrox chips, just switch to RGB16. That will be pretty and fast enough 
for these chips' server systems. Anyone who cares about fast and 
beautiful should buy a real graphics card.

Best regards
Thomas

> 
> 
> Thanks,
> pq
> 
>>> + *     On most hardware, VRAM read access are slow, so when doing the software
>>> + *     conversion, the dumb buffer should be allocated in system RAM in order to
>>> + *     have decent performance.
>>> + *     Extra care should be taken when doing software conversion with
>>> + *     DRM_CAP_DUMB_PREFER_SHADOW, there are more detailed explanations here:
>>> + *     https://lore.kernel.org/dri-devel/20230818162415.2185f8e3@eldfell/
>>>     */
>>>    
>>>    static unsigned int drm_num_planes(struct drm_device *dev)
>>>
>>> base-commit: 82d750e9d2f5d0594c8f7057ce59127e701af781
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
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