Re: [PATCH 2/3] drm/i915: Protect dev_priv->atomic_cdclk_freq with all the crtc locks

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

On Tue, Nov 15, 2016 at 02:53:00PM +0100, Maarten Lankhorst wrote:
> Op 15-11-16 om 14:41 schreef Ville Syrjälä:
> > On Tue, Nov 15, 2016 at 11:14:29AM +0100, Maarten Lankhorst wrote:
> >> Op 14-11-16 om 17:35 schreef ville.syrjala@linux.intel.com:
> >>> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >>>
> >>> A modeset on one pipe can update dev_priv->atomic_cdclk_freq without
> >>> actually touching the hardware, in which case we won't force a modeset
> >>> on all the pipes, and thus won't lock any of the other pipes either.
> >>> That means a parallel plane update on another pipe could be looking at
> >>> a stale dev_priv->atomic_cdcdlk_freq and thus fail to notice when the
> >>> plane configuration is invalid, or potentially reject a valid update.
> >>>
> >>> To overcome this we must protect writes to atomic_cdclk_freq with
> >>> all the crtc locks, and thus for reads any single crtc lock will
> >>> be sufficient protection.
> >>>
> >>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >>> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >>> ---
> >>>  drivers/gpu/drm/i915/i915_drv.h      |  9 +++++++-
> >>>  drivers/gpu/drm/i915/intel_display.c | 41 +++++++++++++++++++++++++++++++-----
> >>>  2 files changed, 44 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> >>> index c0f1dfc7119e..66d2950dc657 100644
> >>> --- a/drivers/gpu/drm/i915/i915_drv.h
> >>> +++ b/drivers/gpu/drm/i915/i915_drv.h
> >>> @@ -1874,7 +1874,14 @@ struct drm_i915_private {
> >>>  
> >>>  	unsigned int fsb_freq, mem_freq, is_ddr3;
> >>>  	unsigned int skl_preferred_vco_freq;
> >>> -	unsigned int cdclk_freq, max_cdclk_freq, atomic_cdclk_freq;
> >>> +	unsigned int cdclk_freq, max_cdclk_freq;
> >>> +
> >>> +	/*
> >>> +	 * For reading holding any crtc lock is sufficient,
> >>> +	 * for writing must hold all of them.
> >>> +	 */
> >>> +	unsigned int atomic_cdclk_freq;
> >>> +
> >>>  	unsigned int max_dotclk_freq;
> >>>  	unsigned int rawclk_freq;
> >>>  	unsigned int hpll_freq;
> >>> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> >>> index 70f3f0b70263..d7a4bc63b05b 100644
> >>> --- a/drivers/gpu/drm/i915/intel_display.c
> >>> +++ b/drivers/gpu/drm/i915/intel_display.c
> >>> @@ -13946,13 +13946,32 @@ static int haswell_mode_set_planes_workaround(struct drm_atomic_state *state)
> >>>  	return 0;
> >>>  }
> >>>  
> >>> +static int intel_lock_all_pipes(struct drm_atomic_state *state)
> >>> +{
> >>> +	struct drm_crtc *crtc;
> >>> +
> >>> +	/* Add all pipes to the state */
> >>> +	for_each_crtc(state->dev, crtc) {
> >>> +		struct drm_crtc_state *crtc_state;
> >>> +
> >>> +		crtc_state = drm_atomic_get_crtc_state(state, crtc);
> >>> +		if (IS_ERR(crtc_state))
> >>> +			return PTR_ERR(crtc_state);
> >>> +	}
> >>> +
> >>> +	return 0;
> >>> +}
> >>> +
> >>>  static int intel_modeset_all_pipes(struct drm_atomic_state *state)
> >>>  {
> >>>  	struct drm_crtc *crtc;
> >>>  	struct drm_crtc_state *crtc_state;
> >>>  	int ret = 0;
> >>>  
> >>> -	/* add all active pipes to the state */
> >>> +	/*
> >>> +	 * Add all pipes to the state, and force
> >>> +	 * a modeset on all the active ones.
> >>> +	 */
> >>>  	for_each_crtc(state->dev, crtc) {
> >>>  		crtc_state = drm_atomic_get_crtc_state(state, crtc);
> >>>  		if (IS_ERR(crtc_state))
> >>> @@ -14018,12 +14037,24 @@ static int intel_modeset_checks(struct drm_atomic_state *state)
> >>>  		if (ret < 0)
> >>>  			return ret;
> >>>  
> >>> +		/*
> >>> +		 * Writes to dev_priv->atomic_cdclk_freq must protected by
> >>> +		 * holding all the crtc locks, even if we don't end up
> >>> +		 * touching the hardware
> >>> +		 */
> >>> +		if (intel_state->cdclk != dev_priv->atomic_cdclk_freq) {
> >>> +			ret = intel_lock_all_pipes(state);
> >>> +			if (ret < 0)
> >>> +				return ret;
> >>> +		}
> >>> +
> >> Would it be terrible to just use intel_modeset_all_pipes here? Since this can only be different in the all crtc's disabled case
> >> it won't matter much.
> > Is there any benefit in doing that? A bit confusing IMO to force a
> > modeset when you don't have to.
> >
> The case where atomic cdclk changes, but dev_cdclk stays the same can only happen
> if you configure a crtc, but all crtc's stay !active. In all other cases dev_cdclk
> will change too.
> 
> intel_modeset_all_pipes will only set mode_changed on active crtc's, but it will
> add all crtc's to the atomic state regardless to make sure the cdclk stays consistent.

I still don't see what the benefit is. IMO it's just confusing to say
that we're going to force a modeset on a disabled pipe.

-- 
Ville Syrjälä
Intel OTC
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