Re: [Intel-gfx] [PATCH] drm/i915: Read a shadowed mmio register for ggtt flush

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Nov 09, 2023 at 12:01:26PM -0800, Belgaumkar, Vinay wrote:
> 
> On 11/9/2023 11:30 AM, Ville Syrjälä wrote:
> > On Thu, Nov 09, 2023 at 11:21:48AM -0800, Vinay Belgaumkar wrote:
> >> We read RENDER_HEAD as a part of the flush. If GT is in
> >> deeper sleep states, this could lead to read errors since we are
> >> not using a forcewake. Safer to read a shadowed register instead.
> > IIRC shadowing is only thing for writes, not reads.
> 
> Sure, but reading from a shadowed register does return the cached value

Does it? I suppose that would make some sense, but I don't recall that
ever being stated anywhere. At least before the shadow registers
existed reads would just give you zeroes when not awake.

> (even though we don't care about the vakue here). When GT is in deeper 
> sleep states, it is better to read a shadowed (cached) value instead of 
> trying to attempt an mmio register read without a force wake anyways.

So you're saying reads from non-shadowed registers fails somehow
when not awake? How exactly do they fail? And when reading from
a shadowed register that failure never happens?

> 
> Thanks,
> 
> Vinay.
> 
> >
> >> Cc: John Harrison <john.c.harrison@intel.com>
> >> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> >> Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/gt/intel_gt.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/gt/intel_gt.c b/drivers/gpu/drm/i915/gt/intel_gt.c
> >> index ed32bf5b1546..ea814ea5f700 100644
> >> --- a/drivers/gpu/drm/i915/gt/intel_gt.c
> >> +++ b/drivers/gpu/drm/i915/gt/intel_gt.c
> >> @@ -451,7 +451,7 @@ void intel_gt_flush_ggtt_writes(struct intel_gt *gt)
> >>   
> >>   		spin_lock_irqsave(&uncore->lock, flags);
> >>   		intel_uncore_posting_read_fw(uncore,
> >> -					     RING_HEAD(RENDER_RING_BASE));
> >> +					     RING_TAIL(RENDER_RING_BASE));
> >>   		spin_unlock_irqrestore(&uncore->lock, flags);
> >>   	}
> >>   }
> >> -- 
> >> 2.38.1

-- 
Ville Syrjälä
Intel