Re: Defining polarity and trigger mode for static interrupts in _PRT

From: Marc Zyngier <marc.zyngier@arm.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Duc Dang <dhdang@apm.com>, Rafael Wysocki <rafael@kernel.org>,
	linux-pci@vger.kernel.org, linux-acpi@vger.kernel.org,
	patches <patches@apm.com>,
	bhelgaas@google.com, punit.agrawal@arm.com
Subject: Re: Defining polarity and trigger mode for static interrupts in _PRT
Date: Wed, 24 Aug 2016 21:30:05 +0100	[thread overview]
Message-ID: <20160824213005.1a9300ef@arm.com> (raw)
In-Reply-To: <20160824193000.GE23914@localhost>

On Wed, 24 Aug 2016 14:30:00 -0500
Bjorn Helgaas <helgaas@kernel.org> wrote:

> On Wed, Aug 24, 2016 at 03:27:23PM +0100, Lorenzo Pieralisi wrote:
> > [ +Bjorn, Punit]
> > 
> > On Wed, Aug 24, 2016 at 04:06:13AM -0700, Duc Dang wrote:  
> > > [Resend in plain text mode]
> > > 
> > > Hi Lorenzo, Rafael,
> > > 
> > > ACPI 6.1 spec does not specify how to set interrupt polarity and
> > > trigger mode in _PRT when the interrupts are static (hardwired to
> > > specific interrupt inputs in interrupt controller). In current
> > > acpi_pci_irq_enable (drivers/acpi/pci_irq.c) implementation, by
> > > default the trigger mode is set to LEVEL_SENSITIVE, polarity is set to
> > > ACTIVE_LOW. This default setting won't work for ARM64 GICv2, GICv2m,
> > > GICv3 controllers and will cause failures in PCIe AER, PME services
> > > (on X-Gene platforms).  
> 
> PCI (not PCIe) r3.0, sec 2.2.6, says "Interrupts on PCI are optional
> and defined as 'level sensitive,' asserted low."
> 
> I've heard before that ARM64 does this differently, but I still don't
> understand the difference.  Obviously if you plug a legacy PCI card
> into an ARM64 system, it's still going to pull INTA# low to assert an
> interrupt.  So is there something special about ARM64 that inverts
> that, or what?

There is certainly an inverter somewhere on the interrupt path, because
the GIC triggers on level high, not level low. But I don't think that's
the issue Duc is trying to outline here, because that's not something
SW can fix. I'm worried that in his system, the interrupt is edge
triggered instead. 

> 
> > > Is there any way to specify polarity and trigger mode for static
> > > interrupts in _PRT?   
> 
> There is no way I'm aware of in _PRT to specify polarity and trigger
> mode.  I don't know the history, but my guess is that it would be seen
> as superfluous given that the PCI spec requires level, active low.
> 
> Obviously I'm missing something important.

Same here, unless the HW is not PCI compliant...

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.