Re: [PATCH 2/3] irqchip/gic-v3: Detect LPI invalidation MMIO registers

From: Marc Zyngier <maz@kernel.org>
To: Andre Przywara <andre.przywara@arm.com>
Cc: linux-kernel@vger.kernel.org,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Eric Auger <eric.auger@redhat.com>,
	Oliver Upton <oupton@google.com>
Subject: Re: [PATCH 2/3] irqchip/gic-v3: Detect LPI invalidation MMIO registers
Date: Wed, 16 Mar 2022 15:36:54 +0000	[thread overview]
Message-ID: <8735jhzz6x.wl-maz@kernel.org> (raw)
In-Reply-To: <20220316145141.44d20486@slackpad.lan>

On Wed, 16 Mar 2022 14:51:58 +0000,
Andre Przywara <andre.przywara@arm.com> wrote:
> 
> On Tue, 15 Mar 2022 16:50:33 +0000
> Marc Zyngier <maz@kernel.org> wrote:
> 
> Hi,
> 
> > Since GICv4.1, an implementation can offer the same MMIO-based
> > implementation as DirectLPI, only with an ITS. Given that this
> > can be hugely beneficial for workloads that are very LPI masking
> > heavy (although these workloads are admitedly a bit odd).
> > 
> > Interestingly, this is independent of RVPEI, which only *implies*
> > the functionnality.
> > 
> > So let's detect whether the implementation has GICR_CTLR.IR set,
> > and propagate this as DirectLPI to the ITS driver.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  drivers/irqchip/irq-gic-v3.c       | 15 +++++++++++----
> >  include/linux/irqchip/arm-gic-v3.h |  2 ++
> >  2 files changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > index 736163d36b13..363bfe172033 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -918,7 +918,11 @@ static int gic_populate_rdist(void)
> >  static int __gic_update_rdist_properties(struct redist_region *region,
> >  					 void __iomem *ptr)
> >  {
> > -	u64 typer = gic_read_typer(ptr + GICR_TYPER);
> > +	u64 typer;
> > +	u32 ctlr;
> > +
> > +	typer = gic_read_typer(ptr + GICR_TYPER);
> > +	ctlr = readl_relaxed(ptr + GICR_CTLR);
> 
> Is there any reason you didn't keep this together? I thought this was
> recommended, in general?

Sorry, keep what together with what?

> 
> >  
> >  	/* Boot-time cleanip */
> >  	if ((typer & GICR_TYPER_VLPIS) && (typer & GICR_TYPER_RVPEID)) {
> > @@ -941,6 +945,7 @@ static int __gic_update_rdist_properties(struct redist_region *region,
> >  	/* RVPEID implies some form of DirectLPI, no matter what the doc says... :-/ */
> >  	gic_data.rdists.has_rvpeid &= !!(typer & GICR_TYPER_RVPEID);
> >  	gic_data.rdists.has_direct_lpi &= (!!(typer & GICR_TYPER_DirectLPIS) |
> > +					   !!(ctlr & GICR_CTLR_IR) |
> 
> So this means that has_direct_lpi is not really correct anymore, as the
> IR bit only covers the INVL and SYNCR registers, not the GICR_SETLPIR
> and GICR_CLRLPIR registers, if I understand the spec correctly?
> 
> But I guess this is nitpicking, as we don't use direct LPIs at all in
> Linux? And I guess the target is lpi_update_config(), which now doesn't
> need the command queue anymore?

Exactly. The history of this crap is convoluted:

The canonical goal of DirectLPI was to support LPIs without an
ITS. Thankfully, this was never implemented. What was implemented by
our HiSi friends was DirectLPI *with* an ITS, which was illegal at the
time, but also the only way to make GICv4.0 work at a reasonable
speed. That's where the direct_lpi boolean comes from.

RVPEI added some more confusion by offering a subset of DirectLPI for
invalidation of vlpis. And then IR was introduced because there is
really no reason not to offer the same service on GICv3.

> 
> Maybe this could be clarified in the commit message?

Sure, can do.

> 
> >  					   gic_data.rdists.has_rvpeid);
> >  	gic_data.rdists.has_vpend_valid_dirty &= !!(typer & GICR_TYPER_DIRTY);
> >  
> > @@ -962,7 +967,11 @@ static void gic_update_rdist_properties(void)
> >  	gic_iterate_rdists(__gic_update_rdist_properties);
> >  	if (WARN_ON(gic_data.ppi_nr == UINT_MAX))
> >  		gic_data.ppi_nr = 0;
> > -	pr_info("%d PPIs implemented\n", gic_data.ppi_nr);
> > +	pr_info("GICv3 features: %d PPIs, %s%s\n",
> 
> I like having that on one line, but it looks a bit odd with the
> trailing comma when we have neither RSS nor DirectLPI.
> What about:
> 	pr_info("GICv3 features: %d PPIs%s%s\n",
> 	gic_data.ppi_nr,
> 	gic_data.has_rss ? ", RSS" : "",
> 	gic_data.rdists.has_direct_lpi ? ", DirectLPI" : "");

Yeah, looks better.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.