Re: [Intel-gfx] [PATCH 5.4.y] drm/i915: Don't use BAR mappings for ring buffers with LLC

[John Harrison <john.c.harrison@intel.com>]

On 3/15/2023 10:57, Greg KH wrote:
> On Wed, Mar 15, 2023 at 10:07:53AM -0700, John Harrison wrote:
>> On 3/15/2023 00:51, Greg KH wrote:
>>> On Mon, Mar 13, 2023 at 07:22:11PM -0700, John.C.Harrison@Intel.com wrote:
>>>> From: John Harrison <John.C.Harrison@Intel.com>
>>>>
>>>> Direction from hardware is that ring buffers should never be mapped
>>>> via the BAR on systems with LLC. There are too many caching pitfalls
>>>> due to the way BAR accesses are routed. So it is safest to just not
>>>> use it.
>>>>
>>>> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
>>>> Fixes: 9d80841ea4c9 ("drm/i915: Allow ringbuffers to be bound anywhere")
>>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>>> Cc: Jani Nikula <jani.nikula@linux.intel.com>
>>>> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
>>>> Cc: intel-gfx@lists.freedesktop.org
>>>> Cc: <stable@vger.kernel.org> # v4.9+
>>>> Tested-by: Jouni Högander <jouni.hogander@intel.com>
>>>> Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
>>>> Link: https://patchwork.freedesktop.org/patch/msgid/20230216011101.1909009-3-John.C.Harrison@Intel.com
>>>> (cherry picked from commit 65c08339db1ada87afd6cfe7db8e60bb4851d919)
>>>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>>>> (cherry picked from commit 85636167e3206c3fbd52254fc432991cc4e90194)
>>>> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/gt/intel_ringbuffer.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>> Also queued up for 5.10.y, you forgot that one :)
>> I'm still working through the backlog of them.
>>
>> Note that these patches must all be applied as a pair. The 'don't use
>> stolen' can be applied in isolation but won't totally fix the problem.
>> However, applying 'don't use BAR mappings' without applying the stolen patch
>> first will results in problems such as the failure to boot that was recently
>> reported and resulted in a revert in one of the trees.
> I do not understand, you only submitted 1 patch here, what is the
> "pair"?
The original patch series was two patches - 
https://patchwork.freedesktop.org/series/114080/. One to not use stolen 
memory and the other to not use BAR mappings. If the anti-BAR patch is 
applied without the anti-stolen patch then the i915 driver will attempt 
to access stolen memory directly which will fail. So both patches must 
be applied and in the correct order to fix the problem of cache aliasing 
when using BAR accesses on LLC systems.

As above, I am working my way through the bunch of 'FAILED patch' 
emails. The what-to-do instructions in those emails explicitly say to 
send the patch individually in reply to the 'FAILED' message rather than 
as part of any original series.

John.