linux-kernel - Re: [PATCH v6 1/7] drm/panthor: Add performance counter uAPI

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Message-ID: <20251216115906.50fe104d@fedora>
Date: Tue, 16 Dec 2025 11:59:06 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Lukas Zapolskas <lukas.zapolskas@....com>
Cc: Liviu Dudau <liviu.dudau@....com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Adrián Larumbe
 <adrian.larumbe@...labora.com>, nd@....com,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, Mihail
 Atanassov <mihail.atanassov@....com>
Subject: Re: [PATCH v6 1/7] drm/panthor: Add performance counter uAPI

On Mon, 15 Dec 2025 17:14:47 +0000
Lukas Zapolskas <lukas.zapolskas@....com> wrote:

> This patch extends the DEV_QUERY ioctl to return information about the
> performance counter setup for userspace, and introduces the new
> ioctl DRM_PANTHOR_PERF_CONTROL in order to allow for the sampling of
> performance counters.
> 
> The new design is inspired by the perf aux ringbuffer [0], with the
> insert and extract indices being mapped to userspace, allowing
> multiple samples to be exposed at any given time. To avoid pointer
> chasing, the sample metadata and block metadata are inline with
> the elements they describe.
> 
> Userspace is responsible for passing in resources for samples to be
> exposed, including the event file descriptor for notification of new
> sample availability, the ringbuffer BO to store samples, and the
> control BO along with the offset for mapping the insert and extract
> indices. Though these indices are only a total of 8 bytes, userspace
> can then reuse the same physical page for tracking the state of
> multiple buffers by giving different offsets from the BO start to
> map them.
> 
> [0]: https://docs.kernel.org/userspace-api/perf_ring_buffer.html
> 
> Co-developed-by: Mihail Atanassov <mihail.atanassov@....com>
> Signed-off-by: Mihail Atanassov <mihail.atanassov@....com>
> Signed-off-by: Lukas Zapolskas <lukas.zapolskas@....com>
> Reviewed-by: Adrián Larumbe <adrian.larumbe@...labora.com>

A couple things pointed out by Adrian have not been fixed, I think (see
below).

> ---
>  include/uapi/drm/panthor_drm.h | 565 +++++++++++++++++++++++++++++++++
>  1 file changed, 565 insertions(+)
> 
> diff --git a/include/uapi/drm/panthor_drm.h b/include/uapi/drm/panthor_drm.h
> index e238c6264fa1..d1a92172e878 100644
> --- a/include/uapi/drm/panthor_drm.h
> +++ b/include/uapi/drm/panthor_drm.h

[...]

> +/**
> + * struct drm_panthor_perf_info - Performance counter interface information
> + *
> + * Structure grouping all queryable information relating to the performance counter
> + * interfaces.
> + */
> +struct drm_panthor_perf_info {
> +	/**
> +	 * @counters_per_block: The number of 8-byte counters available in a block.
> +	 */
> +	__u32 counters_per_block;
> +
> +	/**
> +	 * @sample_header_size: The size of the header struct available at the beginning
> +	 * of every sample.
> +	 */
> +	__u32 sample_header_size;
> +
> +	/**
> +	 * @block_header_size: The size of the header struct inline with the counters for a
> +	 * single block.
> +	 */
> +	__u32 block_header_size;
> +
> +	/**
> +	 * @sample_size: The size of a fully annotated sample, starting with a sample header
> +	 *               of size @sample_header_size bytes, and all available blocks for the current
> +	 *               configuration, each comprised of @counters_per_block 64-bit counters and
> +	 *               a block header of @block_header_size bytes.
> +	 *
> +	 *               The user must use this field to allocate size for the ring buffer. In
> +	 *               the case of new blocks being added, an old userspace can always use
> +	 *               this field and ignore any blocks it does not know about.
> +	 */
> +	__u32 sample_size;
> +
> +	/** @flags: Combination of drm_panthor_perf_feat_flags flags. */
> +	__u32 flags;
> +
> +	/**
> +	 * @supported_clocks: Bitmask of the clocks supported by the GPU.
> +	 *
> +	 * Each bit represents a variant of the enum drm_panthor_perf_clock.
> +	 *
> +	 * For the same GPU, different implementers may have different clocks for the same hardware
> +	 * block. At the moment, up to three clocks are supported, and any clocks that are present
> +	 * will be reported here.
> +	 */
> +	__u32 supported_clocks;
> +
> +	/** @fw_blocks: Number of FW blocks available. */
> +	__u32 fw_blocks;
> +
> +	/** @cshw_blocks: Number of CSHW blocks available. */
> +	__u32 cshw_blocks;
> +
> +	/** @tiler_blocks: Number of tiler blocks available. */
> +	__u32 tiler_blocks;
> +
> +	/** @memsys_blocks: Number of memsys blocks available. */
> +	__u32 memsys_blocks;
> +
> +	/** @shader_blocks: Number of shader core blocks available. */
> +	__u32 shader_blocks;

You need an extra

	__u32 pad;

to have things aligned on 8 bytes.

> +};
> +

[...]

> +
> +/**
> + * struct drm_panthor_perf_ringbuf_control - Struct used to map in the ring buffer control indices
> + *                                           into memory shared between user and kernel.
> + *
> + */
> +struct drm_panthor_perf_ringbuf_control {
> +	/**
> +	 * @extract_idx: The index of the latest sample that was processed by userspace. Only
> +	 *               modifiable by userspace.
> +	 */
> +	__u64 extract_idx;
> +
> +	/**
> +	 * @insert_idx: The index of the latest sample emitted by the kernel. Only modifiable by
> +	 *               modifiable by the kernel.

"modifiable by" repeated twice.

> +	 */
> +	__u64 insert_idx;
> +};