Re: [Intel-gfx] [PATCH v6] drm/i915: handle uncore spinlock when not available

["Coelho, Luciano" <luciano.coelho@intel.com>]

On Thu, 2023-11-30 at 12:21 +0000, Tvrtko Ursulin wrote:
> On 30/11/2023 11:35, Luca Coelho wrote:
> > The uncore code may not always be available (e.g. when we build the
> > display code with Xe), so we can't always rely on having the uncore's
> > spinlock.
> > 
> > To handle this, split the spin_lock/unlock_irqsave/restore() into
> > spin_lock/unlock() followed by a call to local_irq_save/restore() and
> > create wrapper functions for locking and unlocking the uncore's
> > spinlock.  In these functions, we have a condition check and only
> > actually try to lock/unlock the spinlock when I915 is defined, and
> > thus uncore is available.
> > 
> > This keeps the ifdefs contained in these new functions and all such
> > logic inside the display code.
> > 
> > Cc: Tvrtko Ursulin <tvrto.ursulin@intel.com>
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> > Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
> > ---
> > 
> > 
> > In v2:
> > 
> >     * Renamed uncore_spin_*() to intel_spin_*()
> >     * Corrected the order: save, lock, unlock, restore
> > 
> > In v3:
> > 
> >     * Undid the change to pass drm_i915_private instead of the lock
> >       itself, since we would have to include i915_drv.h and that pulls
> >       in a truckload of other includes.
> > 
> > In v4:
> > 
> >     * After a brief attempt to replace this with a different patch,
> >       we're back to this one;
> >     * Pass drm_i195_private again, and move the functions to
> >       intel_vblank.c, so we don't need to include i915_drv.h in a
> >       header file and it's already included in intel_vblank.c;
> > 
> > In v5:
> > 
> >     * Remove stray include in intel_display.h;
> >     * Remove unnecessary inline modifiers in the new functions.
> > 
> > In v6:
> > 
> >     * Just removed the umlauts from Ville's name, because patchwork
> >       didn't catch my patch and I suspect it was some UTF-8 confusion.
> > 
> >   drivers/gpu/drm/i915/display/intel_vblank.c | 49 ++++++++++++++++-----
> >   1 file changed, 39 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
> > index 2cec2abf9746..221fcd6bf77b 100644
> > --- a/drivers/gpu/drm/i915/display/intel_vblank.c
> > +++ b/drivers/gpu/drm/i915/display/intel_vblank.c
> > @@ -265,6 +265,30 @@ int intel_crtc_scanline_to_hw(struct intel_crtc *crtc, int scanline)
> >   	return (scanline + vtotal - crtc->scanline_offset) % vtotal;
> >   }
> >   
> > +/*
> > + * The uncore version of the spin lock functions is used to decide
> > + * whether we need to lock the uncore lock or not.  This is only
> > + * needed in i915, not in Xe.
> > + *
> > + * This lock in i915 is needed because some old platforms (at least
> > + * IVB and possibly HSW as well), which are not supported in Xe, need
> > + * all register accesses to the same cacheline to be serialized,
> > + * otherwise they may hang.
> > + */
> > +static void intel_vblank_section_enter(struct drm_i915_private *i915)
> > +{
> > +#ifdef I915
> > +	spin_lock(&i915->uncore.lock);
> > +#endif
> > +}
> > +
> > +static void intel_vblank_section_exit(struct drm_i915_private *i915)
> > +{
> > +#ifdef I915
> > +	spin_unlock(&i915->uncore.lock);
> > +#endif
> > +}
> > +
> >   static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
> >   				     bool in_vblank_irq,
> >   				     int *vpos, int *hpos,
> > @@ -302,11 +326,12 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
> >   	}
> >   
> >   	/*
> > -	 * Lock uncore.lock, as we will do multiple timing critical raw
> > -	 * register reads, potentially with preemption disabled, so the
> > -	 * following code must not block on uncore.lock.
> > +	 * Enter vblank critical section, as we will do multiple
> > +	 * timing critical raw register reads, potentially with
> > +	 * preemption disabled, so the following code must not block.
> >   	 */
> > -	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
> > +	local_irq_save(irqflags);
> > +	intel_vblank_section_enter(dev_priv);
> 
> Shouldn't local_irq_save go into intel_vblank_section_enter()? It seems 
> all callers from both i915 and xe end up doing that anyway and naming 
> "vblank_start" was presumed there would be more to the section than 
> cacheline mmio bug. I mean that there is some benefit from keeping the 
> readout timings tight.
> 

The reason is that there is one caller that has already disabled
interrupts when this function is called (see below), so we shouldn't do
it again. 


> >   
> >   	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
> >   
> > @@ -374,7 +399,8 @@ static bool i915_get_crtc_scanoutpos(struct drm_crtc *_crtc,
> >   
> >   	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
> >   
> > -	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
> > +	intel_vblank_section_exit(dev_priv);
> > +	local_irq_restore(irqflags);
> >   
> >   	/*
> >   	 * While in vblank, position will be negative
> > @@ -412,9 +438,13 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
> >   	unsigned long irqflags;
> >   	int position;
> >   
> > -	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
> > +	local_irq_save(irqflags);
> > +	intel_vblank_section_enter(dev_priv);
> > +
> >   	position = __intel_get_crtc_scanline(crtc);
> > -	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
> > +
> > +	intel_vblank_section_exit(dev_priv);
> > +	local_irq_restore(irqflags);
> >   
> >   	return position;
> >   }
> > @@ -537,7 +567,7 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state,
> >   	 * Need to audit everything to make sure it's safe.
> >   	 */
> >   	spin_lock_irqsave(&i915->drm.vblank_time_lock, irqflags);
> > -	spin_lock(&i915->uncore.lock);
> > +	intel_vblank_section_enter(i915);

Here.

--
Cheers,
Luca.