Re: [PATCH v2] drm/panthor: dummy read to discard the stale first GPU_TIMESTAMP read

[Liviu Dudau <liviu.dudau@arm.com>]

On Wed, Sep 23, 2026 at 06:24:48PM +0800, ryan.zhang@oss.nxp.com wrote:
> From: Ryan Zhang <ryan.zhang@oss.nxp.com>
> 
> The first read of GPU_TIMESTAMP after the GPU has been idle can return a
> value that lags the real counter, while a read issued immediately after is
> accurate.
> 
> Userspace correlates this timestamp with the host clock, so a stale value
> shows up as skew between the device and host time domains and makes
> dEQP-VK.pipeline.monolithic.timestamp.calibrated.calibration_test fail
> whenever a query happens to hit a large first-read error.
> 
> Signed-off-by: Ryan Zhang <ryan.zhang@oss.nxp.com>
> Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Pushed to drm-misc-next.

Best regards,
Liviu

> ---
> v2:
>  - Added the r-b in commit msg.
>  - Rebased on drm-misc-next to resolve conflict.
>  - Moved the dummy read into panthor_gpu_get_timestamp() to align with
>    the refactored timestamp query code on drm-misc-next.
> Note: The original v1 patch was based on v6.18 and applied cleanly there.
> Could we also port v1 into 6.18?
> 
>  drivers/gpu/drm/panthor/panthor_gpu.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
> index a383b04f101e..b8df5663f04e 100644
> --- a/drivers/gpu/drm/panthor/panthor_gpu.c
> +++ b/drivers/gpu/drm/panthor/panthor_gpu.c
> @@ -2,6 +2,7 @@
>  /* Copyright 2018 Marty E. Plummer <hanetzer@startmail.com> */
>  /* Copyright 2019 Linaro, Ltd., Rob Herring <robh@kernel.org> */
>  /* Copyright 2019 Collabora ltd. */
> +/* Copyright 2026 NXP */
>  
>  #include <linux/bitfield.h>
>  #include <linux/bitmap.h>
> @@ -470,6 +471,9 @@ void panthor_gpu_resume(struct panthor_device *ptdev)
>  
>  u64 panthor_gpu_get_timestamp(struct panthor_device *ptdev)
>  {
> +	/* Dummy read to avoid stale values. */
> +	gpu_read64_counter(ptdev->gpu->iomem, GPU_TIMESTAMP);
> +
>  	return gpu_read64_counter(ptdev->gpu->iomem, GPU_TIMESTAMP);
>  }
>  
> -- 
> 2.43.0
> 

-- 
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