Does gbm_bo_map() implicitly synchronise?

[Christian König]

Am 18.06.24 um 07:01 schrieb Pierre Ossman:
> On 17/06/2024 20:18, Christian König wrote:
>> Am 17.06.24 um 19:18 schrieb Pierre Ossman:
>>> On 17/06/2024 18:09, Michel Dänzer wrote:
>>>>>
>>>>> Can I know whether it is needed or not? Or should I be cautious 
>>>>> and always do it?
>>>>
>>>> Assuming GBM in the X server uses the GPU HW driver, I'd say it 
>>>> shouldn't be needed.
>>>>
>>>
>>> It does not (except the driver libgbm loads). We're trying to use 
>>> this in Xvnc, so it's all CPU. We're just trying to make sure the 
>>> applications can use the full power of the GPU to render their stuff 
>>> before handing it over to the X server. :)
>>
>> That whole approach won't work.
>>
>> When you don't have a HW driver loaded or at least tell the client 
>> that it should render into a linear buffer somehow then the data in 
>> the buffer will be tilled in a hw specific format.
>>
>> As far as I know you can't read that vendor agnostic with the CPU, 
>> you need the hw driver for that.
>>
>
> I'm confused. What's the goal of the GBM abstraction and specifically 
> gbm_bo_map() if it's not a hardware-agnostic way of accessing buffers?

There is no hardware agnostic way of accessing buffers which contain hw 
specific data.

You always need a hw specific backend for that or use the linear flag 
which makes the data hw agnostic.

>
> In practice, we are getting linear buffers. At least on Intel and AMD 
> GPUs. Nvidia are being a bit difficult getting GBM working, so we 
> haven't tested that yet.

That's either because you have a linear buffer for some reason or the 
hardware specific gbm backend has inserted a blit as Michel described.

> I see there is the GBM_BO_USE_LINEAR flag. We have not used it yet, as 
> we haven't seen a need for it. What is the effect of that? Would it 
> guarantee what we are just lucky to see at the moment?

Michel and/or Marek need to answer that. I'm coming from the kernel side 
and maintaining the DMA-buf implementation backing all this, but I'm not 
an expert on gbm.

Regards,
Christian.

>
> Regards