[Intel-gfx] [PATCH 1/5] drm: Add atomic helper to redo a modeset on current mode

[Daniel Vetter]

On Sat, Oct 22, 2016 at 10:47:25AM +0200, Daniel Vetter wrote:
> On Fri, Oct 21, 2016 at 04:45:39PM -0700, Manasi Navare wrote:
> > This function provides a way for the driver to redo a
> > modeset on the current mode and retry the link training
> > at a lower link rate/lane count/bpp. This will get called
> > incase the link training fails during the current modeset.
> > 
> > Cc: dri-devel at lists.freedesktop.org
> > Cc: Jani Nikula <jani.nikula at linux.intel.com>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> > Signed-off-by: Manasi Navare <manasi.d.navare at intel.com>
> > ---
> >  drivers/gpu/drm/drm_atomic_helper.c | 58 +++++++++++++++++++++++++++++++++++++
> >  include/drm/drm_atomic_helper.h     |  1 +
> >  2 files changed, 59 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index f936276..0c1614e 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -2895,6 +2895,64 @@ int drm_atomic_helper_connector_dpms(struct drm_connector *connector,
> >  EXPORT_SYMBOL(drm_atomic_helper_connector_dpms);
> >  
> >  /**
> > + * drm_atomic_helper_connector_modeset - Force a modeset on a connector
> > + * @connector: DRM connector
> > + *
> > + * Provides a way to redo a modeset with the current mode so that it can
> > + * drop the bpp, link rate/lane count and retry the link training.
> > + *
> > + * Returns:
> > + * Returns 0 on success, negative errno numbers on failure.
> > + */
> > +int
> > +drm_atomic_helper_connector_modeset(struct drm_connector *connector)
> > +{
> > +	struct drm_device *dev = connector->dev;
> > +	struct drm_modeset_acquire_ctx ctx;
> > +	struct drm_atomic_state *state;
> > +	struct drm_connector_state *connector_state;
> > +	struct drm_crtc_state *crtc_state;
> > +	int ret = 0;
> > +
> > +	drm_modeset_acquire_init(&ctx, 0);
> > +	state = drm_atomic_state_alloc(dev);
> > +	if (!state) {
> > +		ret = -ENOMEM;
> > +		goto fail;
> > +	}
> > +	state->acquire_ctx = &ctx;
> > +retry:
> > +	ret = 0;
> > +	connector_state = drm_atomic_get_connector_state(state, connector);
> > +	if (IS_ERR(connector_state)) {
> > +		ret = PTR_ERR(connector_state);
> > +		goto fail;
> > +	}
> > +	if (!connector_state->crtc)
> > +		goto fail;
> > +
> > +	crtc_state = drm_atomic_get_existing_crtc_state(state,
> > +							connector_state->crtc);
> > +	crtc_state->connectors_changed = true;
> 
> This line here only works if the driver uses the helpers for checking and
> commit, and when it doesn't overwrite connectors_changed. I think the
> kerneldoc should mention this.
> 
> The other issue: You cannot call this from modeset code itself because of
> recursion. I think this also must be mentioned.

And if this indeed does a modeset then we have again the same issue as
with upfront link training when the pipe is supposed to be on: Userspace
can observe the kernel playing tricks because the vblank support will be
temporarily disabled.

Not sure how to best fix this, but might be good to grab the crtc->mutex
in the vblank code for stuff like this.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch