http://bugs.freedesktop.org/show_bug.cgi?id=18347
Summary: intelReadPixels needs to wait for hardware
Product: Mesa
Version: unspecified
Platform: Other
OS/Version: All
Status: NEW
Severity: normal
Priority: medium
Component: Drivers/DRI/i915
AssignedTo: [email protected]
ReportedBy: [EMAIL PROTECTED]
Created an attachment (id=20001)
--> (http://bugs.freedesktop.org/attachment.cgi?id=20001)
Patch as described
With the current intel driver, small 1x1 pixel reads often (perhaps even
usually) give the wrong result. I think the problem is that when the
code falls back to _swrast_ReadPixels(), it doesn't wait for the GPU
to finish.
The patch I'll attach here adds an intelFinish() call before calling
_swrast_ReadPixels(). It fixes the problems I'm observing.
Comments:
- I think the intelFlush() call at the beginning of of intelReadPixels()
can be removed. do_blit_readpixels() calls intelFlush() itself,
as does intelFinish(). Performance difference for doing so is
probably minimal.
- intelFinish() does a bit more than is necessary, since it waits for
rendering to all color buffers to finish, not just the one we
are going to read from. Seems unlikely to matter, since presumably
the last thing done before glReadPixels() is to render to the
buffer you are going to read from.
- I don't see where do_blit_readpixels() waits for the blit it emitted
to finish, but I don't have a test case for that code path, and maybe
that happens implicitly or in the upper layers.
- I don't know the code at all, so please double check for sanity :-)
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