linux-kernel - Re: [RFC PATCH v3 2/4] drm: Create DRM_IOCTL_GET

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Message-ID: <20230621110931.6f3c8914@eldfell>
Date:   Wed, 21 Jun 2023 11:09:31 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     André Almeida <andrealmeid@...lia.com>
Cc:     dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
        alexander.deucher@....com, christian.koenig@....com,
        pierre-eric.pelloux-prayer@....com,
        Simon Ser <contact@...rsion.fr>,
        Rob Clark <robdclark@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Daniel Stone <daniel@...ishbar.org>,
        'Marek Olšák' <maraeo@...il.com>,
        Dave Airlie <airlied@...il.com>,
        Michel Dänzer <michel.daenzer@...lbox.org>,
        Samuel Pitoiset <samuel.pitoiset@...il.com>,
        Timur Kristóf <timur.kristof@...il.com>,
        Bas Nieuwenhuizen <bas@...nieuwenhuizen.nl>
Subject: Re: [RFC PATCH v3 2/4] drm: Create DRM_IOCTL_GET_RESET

On Tue, 20 Jun 2023 21:57:17 -0300
André Almeida <andrealmeid@...lia.com> wrote:

> Create a new DRM ioctl operation to get the numbers of resets for a
> given context. The numbers reflect just the resets that happened after
> the context was created, and not since the machine was booted.
> 
> Create a debugfs interface to make easier to test the API without real
> resets.
> 
> Signed-off-by: André Almeida <andrealmeid@...lia.com>
> ---
>  drivers/gpu/drm/drm_debugfs.c |  2 ++
>  drivers/gpu/drm/drm_ioctl.c   | 58 +++++++++++++++++++++++++++++++++++
>  include/drm/drm_device.h      |  3 ++
>  include/drm/drm_drv.h         |  3 ++
>  include/uapi/drm/drm.h        | 21 +++++++++++++
>  include/uapi/drm/drm_mode.h   | 15 +++++++++
>  6 files changed, 102 insertions(+)

...

> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index a87bbbbca2d4..a84559aa0d77 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -1169,6 +1169,27 @@ extern "C" {
>   */
>  #define DRM_IOCTL_MODE_GETFB2		DRM_IOWR(0xCE, struct drm_mode_fb_cmd2)
>  
> +/**
> + * DRM_IOCTL_GET_RESET - Get information about device resets
> + *
> + * This operation requests from the device information about resets. It should
> + * consider only resets that happens after the context is created, therefore,
> + * the counter should be zero during context creation.
> + *
> + * dev_reset_count tells how many resets have happened on this device, and
> + * ctx_reset_count tells how many of such resets were caused by this context.
> + *
> + * Flags can be used to tell if a reset is in progress, and userspace should
> + * wait until it's not in progress anymore to be able to create a new context;
> + * and to tell if the VRAM is considered lost. There's no safe way to clean this
> + * flag so if a context see this flag set, it should be like that until the end
> + * of the context.

Is "this flag" the VRAM_LOST? Or any flag?

Does this mean that not all resets are fatal to the context? Is there
any kind of reset that should not be fatal to a context? All the
rendering APIs seem to assume that any reset is fatal and the context
must be destroyed.

> + */
> +#define DRM_IOCTL_GET_RESET		DRM_IOWR(0xCF, struct drm_get_reset)
> +
> +#define DRM_RESET_IN_PROGRESS	0x1
> +#define DRM_RESET_VRAM_LOST	0x2

Ok, so the dmabuf lost is being communicated here, but how would a
userspace process know on which device a dmabuf resides on?

Let's assume process A uses device 1 to draw, exports a dmabuf, sends
it to process B which imports it to device 2. Device 1 resets and loses
VRAM contents. How would process B notice that the dmabuf is lost when
it never touches device 1 itself?

> +
>  /*
>   * Device specific ioctls should only be in their respective headers
>   * The device specific ioctl range is from 0x40 to 0x9f.
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 43691058d28f..c3257bd1af9c 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -1308,6 +1308,21 @@ struct drm_mode_rect {
>  	__s32 y2;
>  };
>  
> +/**
> + * struct drm_get_reset - Get information about a DRM device resets
> + * @ctx_id: the context id to be queried about resets
> + * @flags: flags
> + * @dev_reset_count: global counter of resets for a given DRM device
> + * @ctx_reset_count: of all the resets counted by this device, how many were
> + * caused by this context.
> + */
> +struct drm_get_reset {
> +	__u32 ctx_id;
> +	__u32 flags;
> +	__u64 dev_reset_count;
> +	__u64 ctx_reset_count;
> +};
> +
>  #if defined(__cplusplus)
>  }
>  #endif

Thanks,
pq

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