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@xxxxxxxxx> > >> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx> > >> Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@xxxxxxxxx> > >> --- > >> 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
