Re: [Intel-gfx] [PATCH] i915/pmu: Use a faster read for 2x32 mmio reads

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

On Thu, 03 Nov 2022 11:07:05 -0700, Umesh Nerlige Ramappa wrote:
>

Hi Umesh,

> 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 in
> getting the GT timestamp. To reduce the latency, define another version
> of the helper that requires caller to acquire uncore->spinlock and
> necessary forcewakes.

Why does this reduces the latency compared to intel_uncore_read64_2x32?

Thanks.
--
Ashutosh

> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> ---
>  .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 26 ++++++++++++++++---
>  drivers/gpu/drm/i915/intel_uncore.h           | 24 +++++++++++++++++
>  2 files changed, 47 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index 693b07a97789..64b0193c9ee4 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -1252,6 +1252,28 @@ static u32 gpm_timestamp_shift(struct intel_gt *gt)
>	return 3 - shift;
>  }
>
> +static u64 gpm_timestamp(struct intel_uncore *uncore, ktime_t *now)
> +{
> +	enum forcewake_domains fw_domains;
> +	u64 reg;
> +
> +	/* Assume MISC_STATUS0 and MISC_STATUS1 are in the same fw_domain */
> +	fw_domains = intel_uncore_forcewake_for_reg(uncore,
> +						    MISC_STATUS0,
> +						    FW_REG_READ);
> +
> +	spin_lock_irq(&uncore->lock);
> +	intel_uncore_forcewake_get__locked(uncore, fw_domains);
> +
> +	reg = intel_uncore_read64_2x32_fw(uncore, MISC_STATUS0, MISC_STATUS1);
> +	*now = ktime_get();
> +
> +	intel_uncore_forcewake_put__locked(uncore, fw_domains);
> +	spin_unlock_irq(&uncore->lock);
> +
> +	return reg;
> +}
> +
>  static void guc_update_pm_timestamp(struct intel_guc *guc, ktime_t *now)
>  {
>	struct intel_gt *gt = guc_to_gt(guc);
> @@ -1261,10 +1283,8 @@ static void guc_update_pm_timestamp(struct intel_guc *guc, ktime_t *now)
>	lockdep_assert_held(&guc->timestamp.lock);
>
>	gt_stamp_hi = upper_32_bits(guc->timestamp.gt_stamp);
> -	gpm_ts = intel_uncore_read64_2x32(gt->uncore, MISC_STATUS0,
> -					  MISC_STATUS1) >> guc->timestamp.shift;
> +	gpm_ts = gpm_timestamp(gt->uncore, now) >> guc->timestamp.shift;
>	gt_stamp_lo = lower_32_bits(gpm_ts);
> -	*now = ktime_get();
>
>	if (gt_stamp_lo < lower_32_bits(guc->timestamp.gt_stamp))
>		gt_stamp_hi++;
> diff --git a/drivers/gpu/drm/i915/intel_uncore.h b/drivers/gpu/drm/i915/intel_uncore.h
> index 5449146a0624..dd0cf7d4ce6c 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.h
> +++ b/drivers/gpu/drm/i915/intel_uncore.h
> @@ -455,6 +455,30 @@ static inline void intel_uncore_rmw_fw(struct intel_uncore *uncore,
>		intel_uncore_write_fw(uncore, reg, val);
>  }
>
> +/*
> + * Introduce a _fw version of intel_uncore_read64_2x32 so that the 64 bit
> + * register read is as quick as possible.
> + *
> + * NOTE:
> + * Prior to calling this function, the caller must
> + * 1. obtain the uncore->lock
> + * 2. acquire forcewakes for the upper and lower register
> + */
> +static inline u64
> +intel_uncore_read64_2x32_fw(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_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);
> +	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)
> --
> 2.36.1
>