linux-kernel - Re: [PATCH v2 1/2] drm/msm/adreno: De-spaghettify the use of memory barriers

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Date: Thu, 27 Jun 2024 02:54:57 +0530
From: Akhil P Oommen <quic_akhilpo@...cinc.com>
To: Daniel Vetter <daniel@...ll.ch>
CC: Konrad Dybcio <konrad.dybcio@...aro.org>, Rob Clark <robdclark@...il.com>,
        Sean Paul <sean@...rly.run>, Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        David Airlie
	<airlied@...il.com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        <linux-arm-msm@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <freedreno@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] drm/msm/adreno: De-spaghettify the use of memory
 barriers

On Wed, Jun 26, 2024 at 09:59:39AM +0200, Daniel Vetter wrote:
> On Tue, Jun 25, 2024 at 08:54:41PM +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 (or drop altogether where possible)
> > 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).
> > 
> > Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> 
> Some in pci these readbacks are actually part of the spec and called
> posting reads. I'd very much recommend drivers create a small wrapper
> function for these cases with a void return value, because it makes the
> code so much more legible and easier to understand.

For Adreno which is configured via mmio, we don't need to do this often. GBIF_HALT
is a scenario where we need to be extra careful as it can potentially cause some
internal lockup. Another scenario I can think of is GPU soft reset where need to
keep a delay on cpu side after triggering. We should closely scrutinize any
other instance that comes up. So I feel a good justification as a comment here
would be enough, to remind the reader. Think of it as a way to discourage the
use by making it hard.

This is a bit subjective, I am fine if you have a strong opinion on this.

-Akhil.

> -Sima
> 
> > ---
> >  drivers/gpu/drm/msm/adreno/a6xx_gmu.c |  4 +---
> >  drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 10 ++++++----
> >  2 files changed, 7 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> > index 0e3dfd4c2bc8..09d640165b18 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
> > @@ -466,9 +466,7 @@ 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));
> >  
> >  	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 c98cdb1e9326..4083d0cad782 100644
> > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > @@ -855,14 +855,16 @@ static int hw_init(struct msm_gpu *gpu)
> >  	/* Clear GBIF halt in case GX domain was not collapsed */
> >  	if (adreno_is_a619_holi(adreno_gpu)) {
> >  		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
> > +		gpu_read(gpu, REG_A6XX_GBIF_HALT);
> > +
> >  		gpu_write(gpu, REG_A6XX_RBBM_GPR0_CNTL, 0);
> > -		/* Let's make extra sure that the GPU can access the memory.. */
> > -		mb();
> > +		gpu_read(gpu, REG_A6XX_RBBM_GPR0_CNTL);
> >  	} else if (a6xx_has_gbif(adreno_gpu)) {
> >  		gpu_write(gpu, REG_A6XX_GBIF_HALT, 0);
> > +		gpu_read(gpu, REG_A6XX_GBIF_HALT);
> > +
> >  		gpu_write(gpu, REG_A6XX_RBBM_GBIF_HALT, 0);
> > -		/* Let's make extra sure that the GPU can access the memory.. */
> > -		mb();
> > +		gpu_read(gpu, REG_A6XX_RBBM_GBIF_HALT);
> >  	}
> >  
> >  	/* Some GPUs are stubborn and take their sweet time to unhalt GBIF! */
> > 
> > -- 
> > 2.45.2
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch