[PATCH 00/14] arm_pmu: ACPI support

[Lorenzo Pieralisi]

On Tue, Mar 14, 2017 at 06:47:34PM +0000, Mark Rutland wrote:
> On Tue, Mar 14, 2017 at 11:49:19AM +0000, Mark Rutland wrote:
> > On Fri, Mar 10, 2017 at 04:14:57PM -0600, Jeremy Linton wrote:
> > > I tried these patches on a m400 (which uses PPIs), and the kernel
> > > fails to come up enough to login via the network (which works with
> > > 4.11rc1 without these patches). So, I suspect there is something
> > > wrong with them. 
> > 
> > Indeed; sorry about this. I'll see if I can get access to a board to try
> > local debugging.
> 
> > > About the only thing it says with any meaning when earlycon is passed is:
> > > [   10.965147] Serial: 8250/16550 driver, 32 ports, IRQ sharing enabled
> > > [   11.064193] dw-apb-uart APMC0D08:00: cannot get irq
> > > 
> > > and promptly hangs up.
> 
> I believe that this is a latent FW bug, in a beta FW, exposed by recent
> changes.
> 
> I managed to get access to two APM Mustang boards. I can reproduce the
> issue with a vanilla v4.11-rc1 defconfig on one, which has a beta FW.
> The same kernel boots fine on the other, which has a later released FW.
> 
> I bisected the issue down to commit:
> 
>   d44fa3d46079dc09 ("ACPI: Add support for ResourceSource/IRQ domain mapping")
> 
> It seems the beta FW describes the UART interrupt with an Extended
> Interrupt Descriptor with the Consumer/Producer bit clear. Per the spec,
> this means "This device produces and consumes this resource", which
> doesn't make sense here given the UART is not an interrupt controller.
> 
> The (working) released FW doesn't use an Extended Interrupt Descriptor
> for this interrupt, sidestepping the issue.
> 
> Given that (AFAICT) this only affects a known-broken, beta FW, I don't
> think that we care that much.
> 
> However, I think there is a larger potential issue here.
> 
> In acpi_irq_parse_one_cb(), we skip interrupts with an Extended
> Interrupt Descriptor with the Consumer/Producer bit clear. It sounds
> like devices which behave as interrupt controllers could legitimately
> have this bit set for interrupts they generate and deliver to
> themselves, and we'd erroneously skip these when parsing interrupts.
> 
> It's not entirely clear to me why this bit exists at all, given that
> it's not used as part of mapping the interrupt, and if you really want
> to know you can map the interrupt and look at the result.

It is not entirely clear to anyone (and it has been a source of major
trouble for other descriptors - ie memory - to the point that it has been
deprecated for *some* descriptors). IIRC (Hanjun ?) the only reason why
Agustin added the check for that bit in drivers/acpi/resource.c
(is_gsi()) is to prevent ACPI core code allocating IRQs for the MBIgen
device that, _erroneously_, was using an extended interrupt descriptor
to list IRQ lines (that I have no idea what they represent) in order to
count how many MSI vectors it would have to allocate.

The gist is: removing the check for the extended interrupt descriptor
producer/consumer bit in ACPI code therefore ignoring that bit should
be harmless (given that the MBIgen bindings should be changed anyway).

The meaning of that bit (and how we can flag up a FW bug) it is still
a bit fuzzy IMO.

Agustin, Hanjun, comments ?

Lorenzo