linux-kernel - Re: [PATCH 07/17] drm/msm/adreno: Add fenced regwrite support

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Message-ID: <d696e7df-7f11-4491-89ff-ba71274ae101@oss.qualcomm.com>
Date: Thu, 24 Jul 2025 02:36:30 +0530
From: Akhil P Oommen <akhilpo@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>, Sean Paul <sean@...rly.run>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/17] drm/msm/adreno: Add fenced regwrite support

On 7/22/2025 8:22 PM, Konrad Dybcio wrote:
> On 7/22/25 3:39 PM, Dmitry Baryshkov wrote:
>> On Sun, Jul 20, 2025 at 05:46:08PM +0530, Akhil P Oommen wrote:
>>> There are some special registers which are accessible even when GX power
>>> domain is collapsed during an IFPC sleep. Accessing these registers
>>> wakes up GPU from power collapse and allow programming these registers
>>> without additional handshake with GMU. This patch adds support for this
>>> special register write sequence.
>>>
>>> Signed-off-by: Akhil P Oommen <akhilpo@....qualcomm.com>
>>> ---
>>>  drivers/gpu/drm/msm/adreno/a6xx_gpu.c     | 63 ++++++++++++++++++++++++++++++-
>>>  drivers/gpu/drm/msm/adreno/a6xx_gpu.h     |  1 +
>>>  drivers/gpu/drm/msm/adreno/a6xx_preempt.c | 20 +++++-----
>>>  3 files changed, 73 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>> index 491fde0083a202bec7c6b3bca88d0e5a717a6560..8c004fc3abd2896d467a9728b34e99e4ed944dc4 100644
>>> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>> @@ -16,6 +16,67 @@
>>>  
>>>  #define GPU_PAS_ID 13
>>>  
>>> +static bool fence_status_check(struct msm_gpu *gpu, u32 offset, u32 value, u32 status, u32 mask)
>>> +{
>>> +	/* Success if !writedropped0/1 */
>>> +	if (!(status & mask))
>>> +		return true;
>>> +
>>> +	udelay(10);
>>
>> Why do we need udelay() here? Why can't we use interval setting inside
>> gmu_poll_timeout()?
> 
> Similarly here:
> 
> [...]
> 
>>> +	if (!gmu_poll_timeout(gmu, REG_A6XX_GMU_AHB_FENCE_STATUS, status,
>>> +			fence_status_check(gpu, offset, value, status, mask), 0, 1000))
>>> +		return 0;
>>> +
>>> +	dev_err_ratelimited(gmu->dev, "delay in fenced register write (0x%x)\n",
>>> +			offset);
>>> +
>>> +	/* Try again for another 1ms before failing */
>>> +	gpu_write(gpu, offset, value);
>>> +	if (!gmu_poll_timeout(gmu, REG_A6XX_GMU_AHB_FENCE_STATUS, status,
>>> +			fence_status_check(gpu, offset, value, status, mask), 0, 1000))
>>> +		return 0;
>>> +
>>> +	dev_err_ratelimited(gmu->dev, "fenced register write (0x%x) fail\n",
>>> +			offset);
> 
> We may want to combine the two, so as not to worry the user too much..
> 
> If it's going to fail, I would assume it's going to fail both checks
> (unless e.g. the bus is so congested a single write can't go through
> to a sleepy GPU across 2 miliseconds, but that's another issue)

In case of success, we cannot be sure if the first write went through.
So we should poll separately.

-Akhil.

> 
> Konrad