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

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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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@phytec.de>
> > 
> > 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 | 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?

As far as I can see, they are both doing the same thing (except one
does for all the CRTCs, but I guess that would no longer be needed
with the change you suggested above):
  - drm_crtc_vblank_get
  - Wait for the vblank count to change
  - drm_crtc_vblank_put

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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