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

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

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@oss.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)

Konrad