linux-kernel - Re: [PATCH v2 1/2] drm/panfrost: Add SYSTEM_TIMESTAMP and SYSTEM_TIMESTAMP

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Message-ID: <72ad853a-cb65-42aa-829e-335362d2a4b5@arm.com>
Date: Thu, 15 Aug 2024 15:31:34 +0100
From: Steven Price <steven.price@....com>
To: Mary Guillemard <mary.guillemard@...labora.com>,
 linux-kernel@...r.kernel.org
Cc: dri-devel@...ts.freedesktop.org, kernel@...labora.com,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Rob Herring <robh@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v2 1/2] drm/panfrost: Add SYSTEM_TIMESTAMP and
 SYSTEM_TIMESTAMP_FREQUENCY parameters

On 14/08/2024 12:21, Mary Guillemard wrote:
> Expose system timestamp and frequency supported by the GPU.
> 
> Mali uses an external timer as GPU system time. On ARM, this is wired to
> the generic arch timer so we wire cntfrq_el0 as device frequency.
> 
> This new uAPI will be used in Mesa to implement timestamp queries and
> VK_KHR_calibrated_timestamps.
> 
> v2:
> - Rewrote to use GPU timestamp register
> - Add missing include for arch_timer_get_cntfrq
> - Rework commit message
> 
> Signed-off-by: Mary Guillemard <mary.guillemard@...labora.com>
> ---
>  drivers/gpu/drm/panfrost/panfrost_drv.c  | 35 ++++++++++++++++++++++++
>  drivers/gpu/drm/panfrost/panfrost_gpu.c  | 12 ++++++++
>  drivers/gpu/drm/panfrost/panfrost_gpu.h  |  1 +
>  drivers/gpu/drm/panfrost/panfrost_regs.h |  2 ++
>  include/uapi/drm/panfrost_drm.h          |  2 ++
>  5 files changed, 52 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index 671eed4ad890..83696d06d697 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -3,6 +3,10 @@
>  /* Copyright 2019 Linaro, Ltd., Rob Herring <robh@...nel.org> */
>  /* Copyright 2019 Collabora ltd. */
>  
> +#ifdef CONFIG_ARM_ARCH_TIMER
> +#include <asm/arch_timer.h>
> +#endif
> +
>  #include <linux/module.h>
>  #include <linux/of.h>
>  #include <linux/pagemap.h>
> @@ -24,10 +28,26 @@
>  static bool unstable_ioctls;
>  module_param_unsafe(unstable_ioctls, bool, 0600);
>  
> +static int panfrost_ioctl_query_timestamp(struct panfrost_device *pfdev,
> +					  u64 *arg)
> +{
> +	int ret;
> +
> +	ret = pm_runtime_resume_and_get(pfdev->dev);
> +	if (ret)
> +		return ret;
> +
> +	*arg = panfrost_timestamp_read(pfdev);
> +
> +	pm_runtime_put(pfdev->dev);
> +	return 0;
> +}
> +
>  static int panfrost_ioctl_get_param(struct drm_device *ddev, void *data, struct drm_file *file)
>  {
>  	struct drm_panfrost_get_param *param = data;
>  	struct panfrost_device *pfdev = ddev->dev_private;
> +	int ret;
>  
>  	if (param->pad != 0)
>  		return -EINVAL;
> @@ -69,6 +89,21 @@ static int panfrost_ioctl_get_param(struct drm_device *ddev, void *data, struct
>  		PANFROST_FEATURE_ARRAY(JS_FEATURES, js_features, 15);
>  		PANFROST_FEATURE(NR_CORE_GROUPS, nr_core_groups);
>  		PANFROST_FEATURE(THREAD_TLS_ALLOC, thread_tls_alloc);
> +
> +	case DRM_PANFROST_PARAM_SYSTEM_TIMESTAMP:
> +		ret = panfrost_ioctl_query_timestamp(pfdev, &param->value);
> +		if (ret)
> +			return ret;
> +		break;
> +
> +	case DRM_PANFROST_PARAM_SYSTEM_TIMESTAMP_FREQUENCY:
> +#ifdef CONFIG_ARM_ARCH_TIMER
> +		param->value = arch_timer_get_cntfrq();
> +#else
> +		param->value = 0;
> +#endif
> +		break;
> +
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gpu.c b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> index fd8e44992184..92de71725f53 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gpu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gpu.c
> @@ -380,6 +380,18 @@ unsigned long long panfrost_cycle_counter_read(struct panfrost_device *pfdev)
>  	return ((u64)hi << 32) | lo;
>  }
>  
> +unsigned long long panfrost_timestamp_read(struct panfrost_device *pfdev)
> +{
> +	u32 hi, lo;
> +
> +	panfrost_cycle_counter_get(pfdev);
> +	hi = gpu_read(pfdev, GPU_TIMESTAMP_HI);
> +	lo = gpu_read(pfdev, GPU_TIMESTAMP_LO);

To get a reliable timestamp you need to use the hi-lo-hi trick:

do {
	hi = gpu_read(pfdev, GPU_TIMESTAMP_HI);
	lo = gpu_read(pfdev, GPU_TIMESTAMP_LO);
} while (hi != gpu_read(pfdev, GPU_TIMESTAMP_HI);

Otherwise if you read around the point of overflowing from LO to HI
you'll get a value which is 2^32 out - see panfrost_cycle_counter_read().

I'm also a little concerned that we have a different API here -
panfrost_cycle_counter_read() assumes that the cycle counter has been
enabled by the caller, but this doesn't. It would be nice to keep these
two functions consistent (i.e. move the get/put into
panfrost_ioctl_query_timestamp()).

Steve

> +	panfrost_cycle_counter_put(pfdev);
> +
> +	return ((u64)hi << 32) | lo;
> +}
> +
>  static u64 panfrost_get_core_mask(struct panfrost_device *pfdev)
>  {
>  	u64 core_mask;
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gpu.h b/drivers/gpu/drm/panfrost/panfrost_gpu.h
> index d841b86504ea..b4fef11211d5 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gpu.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_gpu.h
> @@ -20,6 +20,7 @@ void panfrost_gpu_suspend_irq(struct panfrost_device *pfdev);
>  void panfrost_cycle_counter_get(struct panfrost_device *pfdev);
>  void panfrost_cycle_counter_put(struct panfrost_device *pfdev);
>  unsigned long long panfrost_cycle_counter_read(struct panfrost_device *pfdev);
> +unsigned long long panfrost_timestamp_read(struct panfrost_device *pfdev);
>  
>  void panfrost_gpu_amlogic_quirk(struct panfrost_device *pfdev);
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_regs.h b/drivers/gpu/drm/panfrost/panfrost_regs.h
> index c25743b05c55..c7bba476ab3f 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_regs.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_regs.h
> @@ -78,6 +78,8 @@
>  
>  #define GPU_CYCLE_COUNT_LO		0x90
>  #define GPU_CYCLE_COUNT_HI		0x94
> +#define GPU_TIMESTAMP_LO		0x98
> +#define GPU_TIMESTAMP_HI		0x9C
>  
>  #define GPU_THREAD_MAX_THREADS		0x0A0	/* (RO) Maximum number of threads per core */
>  #define GPU_THREAD_MAX_WORKGROUP_SIZE	0x0A4	/* (RO) Maximum workgroup size */
> diff --git a/include/uapi/drm/panfrost_drm.h b/include/uapi/drm/panfrost_drm.h
> index 9f231d40a146..52b050e2b660 100644
> --- a/include/uapi/drm/panfrost_drm.h
> +++ b/include/uapi/drm/panfrost_drm.h
> @@ -172,6 +172,8 @@ enum drm_panfrost_param {
>  	DRM_PANFROST_PARAM_NR_CORE_GROUPS,
>  	DRM_PANFROST_PARAM_THREAD_TLS_ALLOC,
>  	DRM_PANFROST_PARAM_AFBC_FEATURES,
> +	DRM_PANFROST_PARAM_SYSTEM_TIMESTAMP,
> +	DRM_PANFROST_PARAM_SYSTEM_TIMESTAMP_FREQUENCY,
>  };
>  
>  struct drm_panfrost_get_param {