[Mesa-dev] [PATCH 02/10] dri_interface: add __DRI_IMAGE_TRANSFER_USER_STRIDE

[Nicolai Hähnle]

On 27.04.2018 19:21, Gurchetan Singh wrote:
> On Fri, Apr 27, 2018 at 2:00 AM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>> That was the whole point of the USER_STRIDE business. There are two
>> alternatives I can see:
>>
>> 1. Change minigbm so that it always maps the entire texture regardless of
>> what the caller requests. This is a very simple fix, but it has the downside
>> that the driver will always blit the entire surface to staging even if only
>> a single pixel is needed. That can obviously bite us for performance and
>> power, which is why I didn't want to go down that route.
> 
> The DRI backend maps the entire texture currently:
> 
> https://chromium.googlesource.com/chromiumos/platform/minigbm/+/master/dri.c#266
> 
> This is consistent with what other drivers do as well, since GEM
> ioctls (i.e, DRM_IOCTL_I915_GEM_MMAP_GTT) don't support partial
> mappings.  I assume the AMD map ioctl also maps the entire buffer, but
> the shadow buffer is smaller.

I assume that those other drivers don't have to copy pixels as part of 
their map / unmap, so for them mapping the entire texture is okay, since 
there is no performance/power difference either way.

We do have to actually copy pixels during map / unmap, so mapping the 
entire texture is a no-go if we're not actually going to use it all.


>> 2. Instead of having a USER_STRIDE interface, just simplify the story to
>> saying that whenever a transfer allocates a staging texture, it should
>> allocate a texture sized for the entire texture, but perform blits only for
>> the region that the user has requested.
>> This ends up being the same thing in the end, but perhaps it's better as an
>> interface because it's more explicit about what's happening.
>>
>> By the way: the fact that gralloc wants to *cache* mappings will also cause
>> us pain, but that's perhaps something we should really just change in
>> gralloc.
> 
> We modified minigbm to not cache mappings for AMD devices
> (crrev.com/c/990892).

Yes, I see now, thanks for that.

Cheers,
Nicolai


> 
>>
>> Cheers,
>> Nicolai
>>
>> --
>> Lerne, wie die Welt wirklich ist,
>> Aber vergiss niemals, wie sie sein sollte.


-- 
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.