[PATCH 0/9] drm: Support GEM SHMEM fbdev without shadow FB

[Thomas Zimmermann]

Hi Javier

Am 08.03.22 um 10:13 schrieb Javier Martinez Canillas:
> Hello Thomas,
> 
> On 3/3/22 21:58, Thomas Zimmermann wrote:
>> Rework the fbdev deferred-I/O to not interfere with fields of struct
>> page. Make the code more flexible and implement GEM SHMEM mmap on top
>> of it.
>>
>> This patchset removes the need for a special shadow framebuffer for
>> fbdev mmap when using GEM SHMEM. SHMEM pages are now mmap'ed from
>> /dev/fb directly.
>>
> 
> Interesting. I wonder if you have any performance improvements after
> dropping the shadow buffer.
> 
>> Patches 2 and 3 rework the fbdev deferred I/O code. It now allows
>> drivers to have better control of the mmap operations. All references
>> to fields in struct page are gone. The rsp state is help in a
>> separate pageref structure.
>>
> 
> That's a very nice cleanup. This really was a huge layering violation.
>   
>> Patches 4 to 7 provide callbacks an helpers to implement deferred I/O
>> with DRM drivers. Specifically, patch 6 introduces a callback to create
>> a dumb buffer for fbdev. This will be useful for many drivers that
>> currently cannot use generic fbdev emulation because of special placement
>> needs of the BO, such as amdgpu or radeon. The drivers can handle the
>> differences to regular dumb buffers in their new callback implementation.
>>
>> Patch 8 extends the GEM SHMEM memory manager with a new helper for fbdev
>> dumb-buffer creation. The returned BO has it's mmap set up to implement
>> deferred I/O with SHMEM pages. No additional shadow buffer is requires
>> any longer. Many drivers can immediatelly benefit from this change.
>>
>> Patch 9 extends virtgpu to support fbdev dumb buffers. It's build on
>> top of GEM SHMEM, but has some modifications that need to be implemented
>> for fbdev as well.
>>
>> There's no immediate fbdev performance improvement from this patchset.
>> Most of all, it removes unnecessary shadow framebuffers and rsp memcpys.
>> A shadow fb for a FullHD display is ~8 MiB, which we now save. The patches
>> do reduce latency between drawing to the fbdev buffer to displaying
>> on the screen. Watching a video on the fbdev console felt smoother and
>> had less flickering.
>>
> 
> Awesome. And you also answered here the question I had above.

There's no significant improvement in performance.

But when I play Bug Buck Bunny in mplayer's benchmark mode, the 
displayed animation looks smoother than before. The test finishes in 73 
seconds instead of 74. I guess that one-second improvement comes from 
the memcpy that we now save. I use a somewhat recent Core i7 for 
testing. On old hardware, we might get more performance out of it. I can 
really only guess here.

Best regards
Thomas

> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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