Re: [PATCH 1/9] drm/simpledrm: Use fbdev defaults for shadow buffering

[Javier Martinez Canillas <javierm@redhat.com>]

On 3/8/22 10:56, Thomas Zimmermann wrote:
> Hi
> 
> Am 08.03.22 um 10:31 schrieb Javier Martinez Canillas:
>> On 3/3/22 21:58, Thomas Zimmermann wrote:
>>> Don't select shadow buffering for the fbdev console explicitly. The
>>> fbdev emulation's heuristic will enable it for any framebuffer with
>>> .dirty callback.
>>>
>>
>> Indeed it does. Not related to your series but looking at this
>> patch I noticed that drivers/gpu/drm/tiny/bochs.c will be the
>> only driver that sets .prefer_shadow_fbdev after this lands.
>>
>> The driver is using GEM so I wonder if after your series this
>> DRM driver could have a .dirty callback and the field just be
>> dropped? Or there would still be a case where it is needed ?
> Bochs uses VRAM helpers (i.e., TTM). Fbdev and userspace would directly 
> write into that buffer memory without a copy. So the dirty function 
> would be empty.
> 
> Other drivers with VRAM helpers (e.g., hibmc, ast) operate on uncached 
> I/O memory AFAICT. So they set .prefer_shadow, which also affects 
> userspace. Bochs uses cached memory and shouldn't need prefer_shadow. 
> Setting prefer_shadow_fbdev is only there for making the fbdev buffer 
> object evictable from video memory.
> 
> As it stands, using prefer_shadow_fbdev is the cleanest solution, even 
> if bochs is the only user of that field.
> 
> Alternatively, we could make it a requirement that qemu provides enough 
> video memory for bochs to unconditionally pin the fbdev BO there without 
> ever evicting. I guess, that would mean 32 MiB of VRAM at least.
>

I see. Thanks a lot for this explanation.
 
> Best regards
> Thomas
> 

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat