[PATCH 5/5] drm/simpledrm: Synchronize access to GEM BOs

[Javier Martinez Canillas]

On 9/23/22 12:43, Thomas Zimmermann wrote:
> Hi
> 
> Am 23.09.22 um 10:06 schrieb Javier Martinez Canillas:
>> On 9/22/22 15:09, Thomas Zimmermann wrote:
>>> Synchronize CPU access to GEM BOs with other drivers when updating the
>>> screen buffer. Imported buffers might otherwise contain stale data.
>>>
>>
>> Can you please elaborate what the problem is? The framebuffers memory is
>> setup by the firmware and would never come from an imported dma-buf, so
>> could the GEM BOs even be shared with other drivers?
>>
>> Or is this done just for the sake of correctness ?
> 
> This isn't about the scanout buffer that we get from the firmware. This 
> is about synchronizing access to the GEM BO memory buffers. Our BOs' 
> memory buffer are allocated from SHMEM or could be imported via dma-buf. 
> In the latter case, another driver or hardware might modify their 
> content concurrently. We need to synchronize before reading the memory 
> from our CPU. The pattern is
> 
>    1) sync BO memory content via begin_cpu_access()
>    2) blit from BO mem to scanout buffer
>    3) release BO memory via end_cpu_access()
>

Yeah, I got that part. What I was asking is if importing from dma-buf is
a real use case for simpledrm or if this patch was more about making the
driver correct and aligned with what other DRM drivers do?

In any case I agree with the change, it's just the rationale for it that
wasn't clear to me.
-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat