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

[Greg KH <greg@kroah.com>]

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"?