linux-kernel - Re: [PATCH 2/3] drm/tegra: Deliver syncpoint base to user space

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Message-ID: <20131011094352.GB8659@ulmo.nvidia.com>
Date:	Fri, 11 Oct 2013 11:43:52 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Arto Merilainen <amerilainen@...dia.com>
Cc:	tbergstrom@...dia.com, dri-devel@...ts.freedesktop.org,
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] drm/tegra: Deliver syncpoint base to user space

On Wed, Oct 09, 2013 at 02:54:09PM +0300, Arto Merilainen wrote:
> This patch makes the necessary additions to deliver syncpoint base
> to the user space.
> 
> This patch splits the index field in the drm_tegra_get_syncpt structure
> into three separate fields (index, support_base, base_id). This allows
> to keep compatibility over kernel versions:
> - The placing of index field stays constant and therefore the interface
>   should stay compatible with earlier userspace versions.
> - The old index field was input-only and it was not supposed to be read
>   afterwards. Delivering data back in the same field should not introduce
>   regressions to any old user space applications.
> - Old kernels left support_base and base_id fields intact (zero) and
>   hence the new user space applications get the correct response from
>   the kernel (=syncpoint does not have a base).
> 
> Signed-off-by: Arto Merilainen <amerilainen@...dia.com>
> ---
>  drivers/gpu/host1x/drm/drm.c | 2 ++
>  include/uapi/drm/tegra_drm.h | 4 +++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/host1x/drm/drm.c b/drivers/gpu/host1x/drm/drm.c
> index 8c61cee..4251563 100644
> --- a/drivers/gpu/host1x/drm/drm.c
> +++ b/drivers/gpu/host1x/drm/drm.c
> @@ -468,6 +468,8 @@ static int tegra_get_syncpt(struct drm_device *drm, void *data,
>  
>  	syncpt = context->client->syncpts[args->index];
>  	args->id = host1x_syncpt_id(syncpt);
> +	args->support_base = syncpt->base ? 1 : 0;
> +	args->base_id = syncpt->base ? syncpt->base->id : 0;
>  
>  	return 0;
>  }
> diff --git a/include/uapi/drm/tegra_drm.h b/include/uapi/drm/tegra_drm.h
> index 73bde4e..7a79ca5 100644
> --- a/include/uapi/drm/tegra_drm.h
> +++ b/include/uapi/drm/tegra_drm.h
> @@ -61,7 +61,9 @@ struct drm_tegra_close_channel {
>  
>  struct drm_tegra_get_syncpt {
>  	__u64 context;
> -	__u32 index;
> +	__u16 index;
> +	__u8 support_base;
> +	__u8 base_id;
>  	__u32 id;

I don't like this. Can't we just add a separate IOCTL? Something like:

	struct drm_tegra_get_syncpt_base {
		__u64 context;
		__u32 syncpt;
		__u32 base;
	};

Which could be used somewhat like this:

	struct drm_tegra_get_syncpt_base args;

	memset(&args, 0, sizeof(args));
	args.context = context;
	args.syncpt = syncpt;

	err = ioctl(fd, DRM_IOCTL_TEGRA_GET_SYNCPT_BASE, &args);
	if (err < 0) {
		/*
		 * perhaps errno == ENOTSUP if the syncpoint has no
		 * associated base?
		 */
	}

	base = args.base;

Thierry

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