[PATCH 1/1] drm/amdgpu: Read clock counter via MMIO to reduce delay (v4)

[Luben Tuikov]

On 2021-06-30 6:10 a.m., YuBiao Wang wrote:
> [Why]
> GPU timing counters are read via KIQ under sriov, which will introduce
> a delay.
>
> [How]
> It could be directly read by MMIO.
>
> v2: Add additional check to prevent carryover issue.
> v3: Only check for carryover for once to prevent performance issue.
> v4: Add comments of the rough frequency where carryover happens.
>
> Signed-off-by: YuBiao Wang <YuBiao.Wang at amd.com>
> Acked-by: Horace Chen <horace.chen at amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c b/drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c
> index ff7e9f49040e..9355494002a1 100644
> --- a/drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c
> +++ b/drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c
> @@ -7609,7 +7609,7 @@ static int gfx_v10_0_soft_reset(void *handle)
>  
>  static uint64_t gfx_v10_0_get_gpu_clock_counter(struct amdgpu_device *adev)
>  {
> -	uint64_t clock;
> +	uint64_t clock, clock_lo, clock_hi, hi_check;
>  
>  	amdgpu_gfx_off_ctrl(adev, false);
>  	mutex_lock(&adev->gfx.gpu_clock_mutex);
> @@ -7620,8 +7620,15 @@ static uint64_t gfx_v10_0_get_gpu_clock_counter(struct amdgpu_device *adev)
>  			((uint64_t)RREG32_SOC15(SMUIO, 0, mmGOLDEN_TSC_COUNT_UPPER_Vangogh) << 32ULL);
>  		break;
>  	default:
> -		clock = (uint64_t)RREG32_SOC15(SMUIO, 0, mmGOLDEN_TSC_COUNT_LOWER) |
> -			((uint64_t)RREG32_SOC15(SMUIO, 0, mmGOLDEN_TSC_COUNT_UPPER) << 32ULL);
> +		clock_hi = RREG32_SOC15_NO_KIQ(SMUIO, 0, mmGOLDEN_TSC_COUNT_UPPER);
> +		clock_lo = RREG32_SOC15_NO_KIQ(SMUIO, 0, mmGOLDEN_TSC_COUNT_LOWER);
> +		hi_check = RREG32_SOC15_NO_KIQ(SMUIO, 0, mmGOLDEN_TSC_COUNT_UPPER);
> +		/* Carryover happens every 4 Giga time cycles counts which is roughly 42 secs */

I'd rather have put the clock frequency here, rather than some interpretation thereof.
This would make this maintainable in the future should the clock frequency change.
"4 Giga time cycles" isn't a standard expression.

Something like:
"The GFX clock frequency is ..., which sets 32-bit carry over with frequency 42 seconds."

It'll also allow anyone to check the math.

Regards,
Luben

> +		if (hi_check != clock_hi) {
> +			clock_lo = RREG32_SOC15_NO_KIQ(SMUIO, 0, mmGOLDEN_TSC_COUNT_LOWER);
> +			clock_hi = hi_check;
> +		}
> +		clock = (uint64_t)clock_lo | ((uint64_t)clock_hi << 32ULL);
>  		break;
>  	}
>  	mutex_unlock(&adev->gfx.gpu_clock_mutex);