Re: [PATCH v3 2/2] drm/fb-helper: implement ioctl FBIO_WAITFORVSYNC

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Feb 21, 2017 at 11:00:59AM +0100, Stefan Lengfeld wrote:
> Hi Maxime,
> 
> On Wed, Feb 15, 2017 at 05:19:09PM +0100, Maxime Ripard wrote:
> > From: Stefan Christ <s.christ@phytec.de>
> > 
> 
> Maybe change the author here. Only the boilerplate code looks my original
> patch. The real code is your work ;-)
> 
> > Implement legacy framebuffer ioctl FBIO_WAITFORVSYNC in the generic
> > framebuffer emulation driver. Legacy framebuffer users like non kms/drm
> > based OpenGL(ES)/EGL implementations may require the ioctl to
> > synchronize drawing or buffer flip for double buffering. It is tested on
> > the i.MX6.
> > 
> > Code is based on
> >     https://github.com/Xilinx/linux-xlnx/blob/master/drivers/gpu/drm/xilinx/xilinx_drm_fb.c#L196
> > 
> > Signed-off-by: Stefan Christ <s.christ@phytec.de>
> > Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> > ---
> >  drivers/gpu/drm/drm_fb_helper.c | 63 ++++++++++++++++++++++++++++++++++-
> >  include/drm/drm_fb_helper.h     | 12 +++++-
> >  2 files changed, 74 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > index c6de87abaca8..15ee9641c725 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -1240,6 +1240,69 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
> >  EXPORT_SYMBOL(drm_fb_helper_setcmap);
> >  
> >  /**
> > + * drm_fb_helper_ioctl - legacy ioctl implementation
> > + * @info: fbdev registered by the helper
> > + * @cmd: ioctl command
> > + * @arg: ioctl argument
> > + *
> > + * A helper to implement the standard fbdev ioctl. Only
> > + * FBIO_WAITFORVSYNC is implemented for now.
> > + */
> > +int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
> > +			unsigned long arg)
> > +{
> > +	struct drm_fb_helper *fb_helper = info->par;
> > +	struct drm_device *dev = fb_helper->dev;
> > +	struct drm_mode_set *mode_set;
> > +	struct drm_crtc *crtc;
> > +	u32 karg;
> > +	int ret = 0;
> > +
> > +	mutex_lock(&dev->mode_config.mutex);
> > +	if (!drm_fb_helper_is_bound(fb_helper)) {
> > +		ret = -EBUSY;
> > +		goto unlock;
> > +	}
> > +
> > +	switch (cmd) {
> > +	case FBIO_WAITFORVSYNC:
> > +		if (get_user(karg, (__u32 __user *)arg)) {
> > +			ret = -EFAULT;
> > +			goto unlock;
> > +		}
> > +
> > +		if (karg >= fb_helper->crtc_count) {
> > +			ret = -EINVAL;
> > +			goto unlock;
> > +		}
> 
> Ville Syrjälä said [1]:
> 
>     FBIO_WAITFORVSYNC takes the crtc as a parmeter, so I'm not sure we want
>     to do this for all the crtcs. Though what that crtc means for fb is
>     rather poorly defined.
> 
> Don't think it takes the crtc as a parameter in 'arg'. When you look at the
> existing fb_ioctl implementations in the directory drivers/video/fbdev/, the
> argument 'arg' is either ignored or must be '0'.

Have a look at matroxfb.

> 
> Furthmore most exiting userspace code just passes the value "0" as the
> argument. For example DirectFB:
> 
>      static const int zero = 0;
>      [...]
>      if (ioctl( dfb_fbdev->fd, FBIO_WAITFORVSYNC, &zero ))

Again the matrox driver is different. And looks like unichrome might have
done something even more exotic with the argument.

But I do agree that using this with the kms fbdev implementation could
be quite problematic as you can't actually know which crtcs the
fb_helper has picked.

> 
> I see two options here: 
> 
> 1/ Just wait for the first vsync event on the first enabled crtc. This is
>    a good approximation for the old framebuffer API. It has no concept of
>    multiple crtcs with concurrenctly running scan out processes. (Maybe apart
>    from extra vendor implementations). So if there are really more than one active
>    crtcs in the system, bad luck with framebuffer API. There will be tearing.
>    Userspace has to use the new DRM/KMS API.
> 
> 2/ Wait for a single vsync event on all active crtcs as Ville Syrjälä 
>    described here [2] in the optimal implementation.
> 
> Kind regards / Mit freundlichen Grüßen
> 	Stefan Lengfeld
> 
> [1] https://lists.freedesktop.org/archives/dri-devel/2017-February/132617.html
> [2] https://lists.freedesktop.org/archives/dri-devel/2017-February/132820.html
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-- 
Ville Syrjälä
Intel OTC