Re: drm/ssd130x: stale pixels when the GPU renders into the framebuffer

["Maíra Canal" <mcanal@igalia.com>]

Hi,

On 30/09/26 05:32, Thomas Zimmermann wrote:
> Hi
> 
> Am 30.09.26 um 09:48 schrieb Javier Martinez Canillas:
> [...]
>>>> I have a patch implementing begin/end_cpu_access to
>>>> drm_gem_prime_dmabuf_ops, which fixed the issue. However, I'm not sure
>>>> we would like to support this use case upstream because, as you
>>>> mentioned, dumb buffers are usually used for software-rendering only.
>>> Before we fix anything, I think we should talk to someone with
>>> dma-buf/PRIME credentials.  As I outlined, the ideomatic pattern is a
>>> producer-consumer relationship and HW rendering into dumb-buffers is not
>>> supported. Those buffers should have been allocated on the v3d side.
>>>
>>> IMHO we should write down these rules in the PRIME documentation and
>>> (soft-)enforce them in the implementation.
>>>
>> Yeah, I think either drm_gem_fb_begin_cpu_access() needs to also sync
>> exported buffers (what Maíra is proposing as a fix) or there should be
>> documentation of the rules for cross-devices buffer sharing through 
>> PRIME.
> 
> This specific case only happens with v3d and only this driver can know 
> when to flush caches. If we flush caches in begin_cpu_access, we easily 
> end up paying the overhead on all systems.
> 

I believe that's the main issue. We would pay a reasonably big price to
support this uncommon (and maybe even wrong) use case.

P.S.: To be clear, I wasn't proposing a fix. I should have called my
patch a "hack", because although it works, I don't believe we should
support this use case for the reasons stated by Thomas.

Best regards,
- Maíra

> 
>>
>> As mentioned, allocating buffers through v3d and importing into ssd130x
>> (or simpledrm) would work correctly, since the v3d driver already sets
>> bo->base.map_wc = true.
> 
> Hard enforcement would mean that drivers would reject to render to 
> imported buffers.  It is most certainly too late to implement this now. 
> But drivers such as v3d could at least print a warning when users 
> attempt to do it.
> 
> Best regards
> Thomas
> 
> 
>>
>