Re: [Intel-gfx] [PATCH 1/2] i915/uncore: Acquire fw before loop in intel_uncore_read64_2x32

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Mon, 07 Nov 2022 02:13:46 -0800, Tvrtko Ursulin wrote:
>
> On 05/11/2022 00:32, Umesh Nerlige Ramappa wrote:
> > PMU reads the GT timestamp as a 2x32 mmio read and since upper and lower
> > 32 bit registers are read in a loop, there is a latency involved between
> > getting the GT timestamp and the CPU timestamp. As part of the
> > resolution, refactor intel_uncore_read64_2x32 to acquire forcewake and
> > uncore lock prior to reading upper and lower regs.
> >
> > Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_uncore.h | 44 ++++++++++++++++++++---------
> >   1 file changed, 30 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_uncore.h b/drivers/gpu/drm/i915/intel_uncore.h
> > index 5449146a0624..e9e38490815d 100644
> > --- a/drivers/gpu/drm/i915/intel_uncore.h
> > +++ b/drivers/gpu/drm/i915/intel_uncore.h
> > @@ -382,20 +382,6 @@ __uncore_write(write_notrace, 32, l, false)
> >    */
> >   __uncore_read(read64, 64, q, true)
> >   -static inline u64
> > -intel_uncore_read64_2x32(struct intel_uncore *uncore,
> > -			 i915_reg_t lower_reg, i915_reg_t upper_reg)
> > -{
> > -	u32 upper, lower, old_upper, loop = 0;
> > -	upper = intel_uncore_read(uncore, upper_reg);
> > -	do {
> > -		old_upper = upper;
> > -		lower = intel_uncore_read(uncore, lower_reg);
> > -		upper = intel_uncore_read(uncore, upper_reg);
> > -	} while (upper != old_upper && loop++ < 2);
> > -	return (u64)upper << 32 | lower;
> > -}
> > -
> >   #define intel_uncore_posting_read(...) ((void)intel_uncore_read_notrace(__VA_ARGS__))
> >   #define intel_uncore_posting_read16(...) ((void)intel_uncore_read16_notrace(__VA_ARGS__))
> >   @@ -455,6 +441,36 @@ static inline void intel_uncore_rmw_fw(struct
> > intel_uncore *uncore,
> >		intel_uncore_write_fw(uncore, reg, val);
> >   }
> >   +static inline u64
> > +intel_uncore_read64_2x32(struct intel_uncore *uncore,
> > +			 i915_reg_t lower_reg, i915_reg_t upper_reg)
> > +{
> > +	u32 upper, lower, old_upper, loop = 0;
> > +	enum forcewake_domains fw_domains;
> > +	unsigned long flags;
> > +
> > +	fw_domains = intel_uncore_forcewake_for_reg(uncore, lower_reg,
> > +						    FW_REG_READ);
> > +
> > +	fw_domains |= intel_uncore_forcewake_for_reg(uncore, upper_reg,
> > +						    FW_REG_READ);
> > +
> > +	spin_lock_irqsave(&uncore->lock, flags);
> > +	intel_uncore_forcewake_get__locked(uncore, fw_domains);
> > +
> > +	upper = intel_uncore_read_fw(uncore, upper_reg);
> > +	do {
> > +		old_upper = upper;
> > +		lower = intel_uncore_read_fw(uncore, lower_reg);
> > +		upper = intel_uncore_read_fw(uncore, upper_reg);
> > +	} while (upper != old_upper && loop++ < 2);
> > +
> > +	intel_uncore_forcewake_put__locked(uncore, fw_domains);
>
> I mulled over the fact this no longer applies the put hysteresis, but then
> I saw GuC busyness is essentially the only current caller so thought it
> doesn't really warrant adding a super long named
> intel_uncore_forcewake_put_delayed__locked helper.
>
> Perhaps it would make sense to move this out of static inline, in which
> case it would also be easier to have the hysteresis without needing to
> export any new helpers, but mostly because it does not feel the static
> inline is justified. Sounds an attractive option but it is passable as is.

Yup, copy that. Also see now how this reduces the read latency. And also it
would increase the latency a bit for a different thread trying to do an
uncore read/write since we hold uncore->lock longer but should be ok I
think.

> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Copy that too:

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>

>
> > +	spin_unlock_irqrestore(&uncore->lock, flags);
> > +
> > +	return (u64)upper << 32 | lower;
> > +}
> > +
> >   static inline int intel_uncore_write_and_verify(struct intel_uncore *uncore,
> >						i915_reg_t reg, u32 val,
> >						u32 mask, u32 expected_val)