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

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

On Thu, Nov 03, 2022 at 10:10:14PM -0700, Dixit, Ashutosh wrote:
>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?

Most of the error introduced is between the time we capture GPU and CPU 
timestamps. I believe, with intel_uncore_read64_2x32, the time taken for 
forcewake is also included in that time, so that adds up.

Regards,
Umesh
>
>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
>>