Re: [PATCH 1/9] drm/doc/rfc: i915 DG1 uAPI

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 28/04/2021 16:16, Kenneth Graunke wrote:
On Monday, April 26, 2021 2:38:53 AM PDT Matthew Auld wrote:
+Existing uAPI issues
+====================
+Some potential issues we still need to resolve.
+
+I915 MMAP
+---------
+In i915 there are multiple ways to MMAP GEM object, including mapping the same
+object using different mapping types(WC vs WB), i.e multiple active mmaps per
+object. TTM expects one MMAP at most for the lifetime of the object. If it
+turns out that we have to backpedal here, there might be some potential
+userspace fallout.
+
+I915 SET/GET CACHING
+--------------------
+In i915 we have set/get_caching ioctl. TTM doesn't let us to change this, but
+DG1 doesn't support non-snooped pcie transactions, so we can just always
+allocate as WB for smem-only buffers.  If/when our hw gains support for
+non-snooped pcie transactions then we must fix this mode at allocation time as
+a new GEM extension.
+
+This is related to the mmap problem, because in general (meaning, when we're
+not running on intel cpus) the cpu mmap must not, ever, be inconsistent with
+allocation mode.
+
+Possible idea is to let the kernel picks the mmap mode for userspace from the
+following table:
+
+smem-only: WB. Userspace does not need to call clflush.
+
+smem+lmem: We allocate uncached memory, and give userspace a WC mapping
+for when the buffer is in smem, and WC when it's in lmem. GPU does snooped
+access, which is a bit inefficient.

I think you meant to write something different here.  What I read was:

- If it's in SMEM, give them WC
- If it's in LMEM, give them WC

Presumably one of those should have been something else, since otherwise
you would have written "always WC" :)

It should have been "always WC", sorry for the confusion.

"smem+lmem: We only ever allow a single mode, so simply allocate this as uncached memory, and always give userspace a WC mapping. GPU still does snooped access here(assuming we can't turn it off like on DG1), which is a bit inefficient."


+
+lmem only: always WC
+
+This means on discrete you only get a single mmap mode, all others must be
+rejected. That's probably going to be a new default mode or something like
+that.
+
+Links
+=====
+[1] https://patchwork.freedesktop.org/series/86798/
+
+[2] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5599#note_553791
_______________________________________________
Intel-gfx mailing list
Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/intel-gfx



[Index of Archives]     [AMD Graphics]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux