Hi Vinay, > -----Original Message----- > From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of > Belgaumkar, Vinay > Sent: Thursday, November 9, 2023 5:02 PM > To: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx > Subject: Re: [PATCH] drm/i915: Read a shadowed mmio register for > ggtt flush > > > On 11/9/2023 12:35 PM, Ville Syrjälä wrote: > > 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? > > We could hit problems like the one being addressed here - > https://patchwork.freedesktop.org/series/125356/. ; Reading from a > shadowed register will avoid any needless references(without a wake) to > the MMIO space. Shouldn't hurt to make this change for all gens IMO. The proposed usage looks accurate for the issue described. Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@xxxxxxxxx> --Radhakrishna(RK) Sripada > > Thanks, > > Vinay. > > > > >> 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
