[Mesa-dev] [PATCH 1/9] drm/doc/rfc: i915 DG1 uAPI

[Matthew Auld]

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