On 27.04.2018 19:21, Gurchetan Singh wrote:
On Fri, Apr 27, 2018 at 2:00 AM, Nicolai Hähnle <nhaeh...@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.
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