Re: [PATCH v2 2/2] drm/fb_helper: implement ioctl FBIO_WAITFORVSYNC

[Ville Syrjälä]

On Mon, Feb 13, 2017 at 11:35:18AM +0100, Maxime Ripard wrote:
> Hi Ville,
> 
> On Fri, Feb 10, 2017 at 04:06:05PM +0200, Ville Syrjälä wrote:
> > On Thu, Feb 02, 2017 at 11:31:57AM +0100, Maxime Ripard wrote:
> > > From: Stefan Christ <s.christ@xxxxxxxxx>
> > > 
> > > 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@xxxxxxxxx>
> > > Signed-off-by: Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >  drivers/gpu/drm/drm_fb_helper.c | 55 ++++++++++++++++++++++++++++++++++-
> > >  include/drm/drm_fb_helper.h     |  5 ++-
> > >  2 files changed, 59 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > > index e934b541feea..39a3532e311c 100644
> > > --- a/drivers/gpu/drm/drm_fb_helper.c
> > > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > > @@ -1234,6 +1234,61 @@ 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;
> > > +	unsigned int i;
> > > +	int ret = 0;
> > > +
> > > +	drm_modeset_lock_all(dev);
> > > +	if (!drm_fb_helper_is_bound(fb_helper)) {
> > > +		drm_modeset_unlock_all(dev);
> > > +		return -EBUSY;
> > > +	}
> > > +
> > > +	switch (cmd) {
> > > +	case FBIO_WAITFORVSYNC:
> > > +		for (i = 0; i < fb_helper->crtc_count; i++) {
> > 
> > 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.
> 
> I guess I could just use that index to retrieve only the right CRTC in
> fb_helper->crtc_info.
> 
> > 
> > > +			struct drm_mode_set *mode_set;
> > > +			struct drm_crtc *crtc;
> > > +
> > > +			mode_set = &fb_helper->crtc_info[i].mode_set;
> > > +			crtc = mode_set->crtc;
> > > +
> > > +			/*
> > > +			 * Only call drm_crtc_wait_one_vblank for crtcs that
> > > +			 * are currently enabled.
> > > +			 */
> > > +			if (!crtc->enabled)
> > > +				continue;
> > > +
> > > +			ret = drm_crtc_vblank_get(crtc);
> > > +			if (!ret) {
> > > +				drm_crtc_wait_one_vblank(crtc);
> > > +				drm_crtc_vblank_put(crtc);
> > > +			}
> > 
> > This looks quite sub-optimal. It should rather do something along the
> > lines of what drm_atomic_helper_wait_for_vblanks() does.
> 
> How is that suboptimal?

You're serializing the waits rather than doing them in parallel.

Let's look at a simple three crtc example (|=vblank):

        time -->
CRTC 1:     |     |     |     |
CRTC 2:   |     |     |     |
CRTC 3: |     |     |     |

Your code waits until here:
            |   |   |
            1   2   3
                    ^

Optimal code would wait until here:
            |
            ^
 
-- 
Ville Syrjälä
Intel OTC