Subject: [PATCH] imx-drm: imx-drm-core: add suspend/resume support

From: [email-masked] (Shawn Guo)
Date: Thu, 24 Jul 2014 22:59:36 +0800

On Thu, Jul 24, 2014 at 12:38:59PM +0200, Daniel Vetter wrote:
> > > > +static int imx_drm_suspend(struct device *dev)
> > > > +{
> > > > +	struct drm_device *drm_dev = dev_get_drvdata(dev);
> > > > +	struct drm_connector *connector;
> > > > +
> > > > +	drm_kms_helper_poll_disable(drm_dev);
> > > > +
> > > 
> > > That's ok.
> > > 
> > > > +	drm_modeset_lock_all(drm_dev);
> > > > +	list_for_each_entry(connector, &drm_dev->mode_config.connector_list, head) {
> > > > +		if (connector->funcs->dpms)
> > > > +			connector->funcs->dpms(connector, DRM_MODE_DPMS_OFF);
> > > > +	}
> > > 
> > > Don't touch DPMS state here. See below.
> > 
> > In which case, how else does the hardware get placed into a low power
> > mode on suspend?
> > 
> > DRM has nothing provided, and this is left up to each DRM driver to
> > implement (probably because it tends to be very driver specific.)
> > i915 for example calls the CRTC DPMS function with DRM_MODE_DPMS_OFF.
> > 
> > This provides a similar mechanism, but also informs the connector, any
> > bridge, and encoder associated with the connector as well as the CRTC
> > to place themselves into a low power mode (which is what
> > DRM_MODE_DPMS_OFF should be doing anyway.)
> 
> Well you need to call internal functions to make sure you can restore the
> state again. Not sure any more how that all works with the crtc helpers
> and whether they restore dpms state properly at all. i915 uses something
> completely different nowadays.

Is it okay to do what exynos driver does, i.e. saving/restoring the dpms
state before/after calling connector .dpms function?

        list_for_each_entry(connector, &drm_dev->mode_config.connector_list, head) {
                int old_dpms = connector->dpms;

                if (connector->funcs->dpms)
                        connector->funcs->dpms(connector, DRM_MODE_DPMS_OFF);

                /* Set the old mode back to the connector for resume */
                connector->dpms = old_dpms;
        }

Shawn