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 11:07:05AM -0700, 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 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.
>
>Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>

Not for review, just to demonstrate one of the solutions to a DG1 BAT 
issue

Thanks,
Umesh
>---
> .../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
>