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

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

On Mon, Sep 21, 2026 at 06:29:57PM +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>

Best regards,
Liviu

> ---
>  drivers/gpu/drm/panthor/panthor_drv.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_drv.c b/drivers/gpu/drm/panthor/panthor_drv.c
> index 720bf889096d..956a147dc282 100644
> --- a/drivers/gpu/drm/panthor/panthor_drv.c
> +++ b/drivers/gpu/drm/panthor/panthor_drv.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 */
>  
>  #ifdef CONFIG_ARM_ARCH_TIMER
>  #include <asm/arch_timer.h>
> @@ -772,6 +773,10 @@ static int panthor_query_timestamp_info(struct panthor_device *ptdev,
>  #else
>  	arg->timestamp_frequency = 0;
>  #endif
> +
> +	/* Dummy read to avoid stale values. */
> +	gpu_read64_counter(ptdev, GPU_TIMESTAMP);
> +
>  	arg->current_timestamp = gpu_read64_counter(ptdev, GPU_TIMESTAMP);
>  	arg->timestamp_offset = gpu_read64(ptdev, GPU_TIMESTAMP_OFFSET);
>  
> -- 
> 2.43.0
> 

-- 
====================
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| but they're not |
| giving me the   |
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