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

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Message-ID: <9e48ea8e-b59b-4620-9781-211cc1f7cc07@oss.qualcomm.com>
Date: Tue, 29 Jul 2025 15:01:43 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Akhil P Oommen <akhilpo@....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 7/24/25 6:54 PM, Akhil P Oommen wrote:
> On 7/24/2025 5:16 PM, Konrad Dybcio wrote:
>> On 7/23/25 11:06 PM, Akhil P Oommen wrote:
>>> 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.
>>
>> You're writing to it 2 (outside fence_status_check) + 2*1000/10 (inside)
>> == 202 times, it really better go through..
> 
> For the following sequence:
> 1. write reg1 <- suppose this is dropped
> 2. write reg2 <- and this went through
> 3. Check fence status <- This will show success

What I'm saying is that fence_status_check() does the same write you
execute inbetween the polling calls

Konrad
> 
>>
>> If it's just about the write reaching the GPU, you can write it once and
>> read back the register you've written to, this way you're sure that the
>> GPU can observe the write
> 
> This is a very unique hw behavior. We can't do posted write.
> 
> -Akhil
> 
>>
>> Konrad
>