Re: [PATCH 01/19] drm/core: Add drm_accurate_vblank_count, v4.

[Mario Kleiner <mario.kleiner.de@gmail.com>]

Sorry for the late review, but see below...

On 04/19/2016 09:52 AM, Maarten Lankhorst wrote:
> This function is useful for gen2 intel devices which have no frame
> counter, but need a way to determine the current vblank count without
> racing with the vblank interrupt handler.
>
> intel_pipe_update_start checks if no vblank interrupt will occur
> during vblank evasion, but cannot check whether the vblank handler has
> run to completion. This function uses the timestamps to determine
> when the last vblank has happened, and interpolates from there.
>
> Changes since v1:
> - Take vblank_time_lock and don't use drm_vblank_count_and_time.
> Changes since v2:
> - Don't return time of last vblank.
> Changes since v3:
> - Change pipe to unsigned int. (Ville)
> - Remove unused documentation for tv_ret. (kbuild)
>
> Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Acked-by: David Airlie <airlied@linux.ie> #irc
> ---
>   drivers/gpu/drm/drm_irq.c | 26 ++++++++++++++++++++++++++
>   include/drm/drmP.h        |  1 +
>   2 files changed, 27 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 3c1a6f18e71c..f1bda13562da 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -303,6 +303,32 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
>   	store_vblank(dev, pipe, diff, &t_vblank, cur_vblank);
>   }
>
> +/**
> + * drm_accurate_vblank_count - retrieve the master vblank counter
> + * @crtc: which counter to retrieve
> + *
> + * This function is similar to @drm_crtc_vblank_count but this
> + * function interpolates to handle a race with vblank irq's.
> + */
> +
> +u32 drm_accurate_vblank_count(struct drm_crtc *crtc)
> +{
> +	struct drm_device *dev = crtc->dev;
> +	unsigned int pipe = drm_crtc_index(crtc);
> +	u32 vblank;
> +	unsigned long flags;
> +

This function is rather dangerous to use on any driver that doesn't have 
precise vblank timestamping, or doesn't have the guarantee that hw 
vblank counters (if there are any) and timestamps update exactly at 
leading edge of vblank, so i think we need some WARN() here and maybe 
much less encouraging docs to avoid this being called from incapable kms 
drivers or in general code.

- If the driver doesn't have precise scanoutpos based timestamping each 
call into drm_update_vblank_count from non-irq context will reset the 
vblank timestamps to zero, so clients will only receive invalid 
timestamps if this is frequently used. Also bogus vblank counts

Atm. only i915 Intel, AMD, NVidia desktop for >= NV-50, maybe nouveau 
driven Tegra parts, and some modern Adrenos (msm/mdp-5 - i assume from 
the code?) support this reliably.

- If the drivers scanoutpos timestamps and/or vblank counter don't 
increment at leading edge we will get funny off-by-one problems with 
vblank counters. That's why we normally only call 
drm_update_vblank_count() from vblank irq on such parts - the only safe 
place to avoid off-by-one problems, and limit vblank disable/enable to 
only at most once every 5 seconds to reduce the problems caused by 
off-by-one errors.

Which restricts the list to only the above parts, maybe minus Adreno 
where i don't know if it obeys the "leading edge" rule or not.

So on most SoC's one must not use this function.

WARN_ON(!dev->vblank_disable_immediate, "This function is unsafe on this 
driver.");

would probably prevent the worst abuse, unless drivers lie about 
vblank_disable_immediate. Not sure how much this was checked for msm / 
Adreno? At least drm_vblank_init() only allows vblank_disable_immediate 
if the driver at least implements proper timestamping.

Not sure how much general use this function will have outside Intel 
gen-2 with the restrictions on safe use?

> +	spin_lock_irqsave(&dev->vblank_time_lock, flags);
> +
> +	drm_update_vblank_count(dev, pipe, 0);
> +	vblank = dev->vblank[pipe].count;

Could do vblank = drm_vblank_count(dev, pipe); instead, given that we 
avoid open coding this in most places.

-mario

> +
> +	spin_unlock_irqrestore(&dev->vblank_time_lock, flags);
> +
> +	return vblank;
> +}
> +EXPORT_SYMBOL(drm_accurate_vblank_count);
> +
>   /*
>    * Disable vblank irq's on crtc, make sure that last vblank count
>    * of hardware and corresponding consistent software vblank counter
> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> index 005202ea5900..90527c41cd5a 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -995,6 +995,7 @@ extern void drm_crtc_vblank_off(struct drm_crtc *crtc);
>   extern void drm_crtc_vblank_reset(struct drm_crtc *crtc);
>   extern void drm_crtc_vblank_on(struct drm_crtc *crtc);
>   extern void drm_vblank_cleanup(struct drm_device *dev);
> +extern u32 drm_accurate_vblank_count(struct drm_crtc *crtc);
>   extern u32 drm_vblank_no_hw_counter(struct drm_device *dev, unsigned int pipe);
>
>   extern int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
>
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