Re: [PATCH v3 1/4] drm: Add drm_crtc_has_vblank()

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 20, 2020 at 01:20:48PM +0100, Thomas Zimmermann wrote:
> The new interface drm_crtc_has_vblank() return true if vblanking has
> been initialized for a certain CRTC, or false otherwise. This function
> will be useful for initializing CRTC state.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> ---
>  drivers/gpu/drm/drm_vblank.c | 21 +++++++++++++++++++++
>  include/drm/drm_vblank.h     |  1 +
>  2 files changed, 22 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 1659b13b178c..c20102899411 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -501,6 +501,27 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
>  }
>  EXPORT_SYMBOL(drm_vblank_init);
>  
> +/**
> + * drm_crtc_has_vblank - test if vblanking has been initialized for
> + *                       a CRTC
> + * @crtc: the CRTC
> + *
> + * Drivers may call this function to test if vblank support is
> + * initialized for a CRTC. For most hardware this means that vblanking
> + * can also be enabled on the CRTC.
> + *
> + * Returns:
> + * True if vblanking has been initialized for the given CRTC, false
> + * otherwise.
> + */
> +bool drm_crtc_has_vblank(const struct drm_crtc *crtc)

So making this specific to a CRTC sounds like a good idea. But it's not
the reality, drm_vblank.c assumes that either everything or nothing
supports vblanks.

The reason for dev->num_crtcs is historical baggage, it predates kms by a
few years. For kms drivers the only two valid values are either 0 or
dev->mode_config.num_crtcs. Yes that's an entire different can of worms
that's been irking me since forever (ideally drm_vblank_init would somehow
loose the num_crtcs argument for kms drivers, but some drivers call this
before they've done all the drm_crtc_init calls so it's complicated).

Hence drm_dev_has_vblank as I suggested. That would also allow you to
replace a bunch of if (dev->num_crtcs) checks in drm_vblank.c, which
should help quite a bit in code readability.

Cheers, Daniel

> +{
> +	struct drm_device *dev = crtc->dev;
> +
> +	return crtc->index < dev->num_crtcs;
> +}
> +EXPORT_SYMBOL(drm_crtc_has_vblank);
> +
>  /**
>   * drm_crtc_vblank_waitqueue - get vblank waitqueue for the CRTC
>   * @crtc: which CRTC's vblank waitqueue to retrieve
> diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> index c16c44052b3d..531a6bc12b7e 100644
> --- a/include/drm/drm_vblank.h
> +++ b/include/drm/drm_vblank.h
> @@ -206,6 +206,7 @@ struct drm_vblank_crtc {
>  };
>  
>  int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs);
> +bool drm_crtc_has_vblank(const struct drm_crtc *crtc);
>  u64 drm_crtc_vblank_count(struct drm_crtc *crtc);
>  u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
>  				   ktime_t *vblanktime);
> -- 
> 2.24.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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