On Thu, 2023-11-30 at 13:24 +0000, Tvrtko Ursulin wrote:
> On 30/11/2023 12:26, Coelho, Luciano wrote:
> > 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@xxxxxxxxx>
> > > > Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> > > > Cc: Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx>
> > > > Reviewed-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> > > > Signed-off-by: Luca Coelho <luciano.coelho@xxxxxxxxx>
> > > > ---
> > > >
> > > >
> > > > 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.
>
> Yeah I saw that but with irqsave/restore it is safe to nest. So for me
> it is more a fundamental question which I raise above.
Sure, it should be safe to nest, but it seemed a bit ugly to me.
I can change it, if you prefer, as your point seems valid, but I will
wait to see what Rodrigo says, since he had already given his r-b, lest
we start ping-ponging on this too much.
--
Cheers,
Luca.
