linux-kernel - Re: [PATCH] drm/msm/adreno: De-spaghettify the use of memory barriers

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Date: Wed, 15 May 2024 00:08:49 +0530
From: Akhil P Oommen <quic_akhilpo@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@...aro.org>
CC: Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        "Abhinav
 Kumar" <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov
	<dmitry.baryshkov@...aro.org>,
        Marijn Suijten
	<marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, "Daniel
 Vetter" <daniel@...ll.ch>,
        Rob Clark <robdclark@...omium.org>, <linux-arm-msm@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <freedreno@...ts.freedesktop.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/adreno: De-spaghettify the use of memory barriers

On Wed, May 08, 2024 at 07:46:31PM +0200, Konrad Dybcio wrote:
> Memory barriers help ensure instruction ordering, NOT time and order
> of actual write arrival at other observers (e.g. memory-mapped IP).
> On architectures employing weak memory ordering, the latter can be a
> giant pain point, and it has been as part of this driver.
> 
> Moreover, the gpu_/gmu_ accessors already use non-relaxed versions of
> readl/writel, which include r/w (respectively) barriers.
> 
> Replace the barriers with a readback that ensures the previous writes
> have exited the write buffer (as the CPU must flush the write to the
> register it's trying to read back) and subsequently remove the hack
> introduced in commit b77532803d11 ("drm/msm/a6xx: Poll for GBIF unhalt
> status in hw_init").
> 
> Fixes: b77532803d11 ("drm/msm/a6xx: Poll for GBIF unhalt status in hw_init")
> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> ---
>  drivers/gpu/drm/msm/adreno/a6xx_gmu.c |  5 ++---
>  drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 14 ++++----------
>  2 files changed, 6 insertions(+), 13 deletions(-)

I prefer this version compared to the v2. A helper routine is
unnecessary here because:
1. there are very few scenarios where we have to read back the same
register.
2. we may accidently readback a write only register.

> 
> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> index 0e3dfd4c2bc8..4135a53b55a7 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> @@ -466,9 +466,8 @@ static int a6xx_rpmh_start(struct a6xx_gmu *gmu)
>  	int ret;
>  	u32 val;
>  
> -	gmu_write(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ, 1 << 1);
> -	/* Wait for the register to finish posting */
> -	wmb();
> +	gmu_write(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ, BIT(1));
> +	gmu_read(gmu, REG_A6XX_GMU_RSCC_CONTROL_REQ);

This is unnecessary because we are polling on a register on the same port below. But I think we
can replace "wmb()" above with "mb()" to avoid reordering between read
and write IO instructions.

>  
>  	ret = gmu_poll_timeout(gmu, REG_A6XX_GMU_RSCC_CONTROL_ACK, val,
>  		val & (1 << 1), 100, 10000);
> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> index 973872ad0474..0acbc38b8e70 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> @@ -1713,22 +1713,16 @@ static int hw_init(struct msm_gpu *gpu)
>  	}
>  
>  	/* Clear GBIF halt in case GX domain was not collapsed */
> +	gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);

We need a full barrier here to avoid reordering. Also, lets add a
comment about why we are doing this odd looking sequence.

> +	gpu_read(gpu, REG_A6XX_GBIF_HALT);
>  	if (adreno_is_a619_holi(adreno_gpu)) {
> -		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
>  		gpu_write(gpu, REG_A6XX_RBBM_GPR0_CNTL, 0);
> -		/* Let's make extra sure that the GPU can access the memory.. */
> -		mb();

We need a full barrier here.

> +		gpu_read(gpu, REG_A6XX_RBBM_GPR0_CNTL);
>  	} else if (a6xx_has_gbif(adreno_gpu)) {
> -		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
>  		gpu_write(gpu, REG_A6XX_RBBM_GBIF_HALT, 0);
> -		/* Let's make extra sure that the GPU can access the memory.. */
> -		mb();

We need a full barrier here.

> +		gpu_read(gpu, REG_A6XX_RBBM_GBIF_HALT);
>  	}
>  
> -	/* Some GPUs are stubborn and take their sweet time to unhalt GBIF! */
> -	if (adreno_is_a7xx(adreno_gpu) && a6xx_has_gbif(adreno_gpu))
> -		spin_until(!gpu_read(gpu, REG_A6XX_GBIF_HALT_ACK));
> -

Why is this removed?

-Akhil

>  	gpu_write(gpu, REG_A6XX_RBBM_SECVID_TSB_CNTL, 0);
>  
>  	if (adreno_is_a619_holi(adreno_gpu))
> 
> ---
> base-commit: 93a39e4766083050ca0ecd6a3548093a3b9eb60c
> change-id: 20240508-topic-adreno-a2d199cd4152
> 
> Best regards,
> -- 
> Konrad Dybcio <konrad.dybcio@...aro.org>
>