Date:   Thu, 25 Mar 2021 14:06:43 +0200
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org
Subject: Re: [PATCH RESEND] intel/pinctrl: check capability offset is between
 MMIO region

On Thu, Mar 25, 2021 at 09:46:46AM +0100, Roger Pau Monné wrote:
> On Wed, Mar 24, 2021 at 06:57:12PM +0200, Andy Shevchenko wrote:
> > On Wed, Mar 24, 2021 at 04:13:59PM +0100, Roger Pau Monné wrote:
> > > On Wed, Mar 24, 2021 at 04:22:44PM +0200, Andy Shevchenko wrote:
> > > > On Wed, Mar 24, 2021 at 02:55:15PM +0100, Roger Pau Monné wrote:
> > > > > On Wed, Mar 24, 2021 at 02:58:07PM +0200, Andy Shevchenko wrote:

...

> > > > Unfortunately it does not expose PCI configuration space.
> > > 
> > > Are those regions supposed to be marked as reserved in the memory map,
> > > or that's left to the discretion of the hardware vendor?
> > 
> > I didn't get. The OS doesn't see them and an internal backbone simply drops any
> > IO access to that region.
> 
> I'm not sure I understand the above reply. My question was whether the
> MMIO regions used by the pinctrl device (as fetched from the ACPI DSDT
> table) are supposed belong to regions marked as RESERVED in the
> firmware memory map (ie: either the e820 or the EFI one).

I don't actually know. I guess it should be done in order to have ACPI device
a possibility to claim the resource.

> > > > > Doing something like pci_device_is_present would require a register
> > > > > that we know will never return ~0 unless the device is not present. As
> > > > > said above, maybe we could use REVID to that end?
> > > > 
> > > > Yes, that's good, see above.
> > > > 
> > > > WRT capabilities, if we crash we will see the report immediately on the
> > > > hardware which has such an issue. (It's quite unlikely we will ever have one,
> > > > that's why I consider it's not critical)
> > > 
> > > I would rather prefer to not crash, because I think the kernel should
> > > only resort to crashing when there's no alternative, and here it's
> > > perfectly fine to just print an error message and don't load the
> > > driver.
> > 
> > Are we speaking about real hardware that has an issue? I eagerly want to know
> > what is that beast.
> 
> OK, I'm not going to resend this anymore. I'm happy with just getting
> the first patch in.
> 
> I think you trust the hardware more that I would do, and I also think
> the check added here is very minimal an unintrusive and serves as a
> way to sanitize the data fetched from the hardware in order to prevent
> a kernel page fault if such data turns out to be wrong.
> 
> Taking a reactive approach of requiring a broken piece of hardware to
> exist in order to sanitize a fetched value seems too risky. I could
> add a WARN_ON or similar if you want some kind of splat that's very
> noticeable when this goes wrong but that doesn't end up in a fatal
> kernel page fault.

You found the issue anyway as long as you had a crash, so current code already
proved that it does it work perfectly.

Since I know what hardware this driver is for, I can assure you, that it will
be quite unlikely to have wrong data in the capability register. The data sheet
is crystal clear about the register's contents: on real hardware it must be
present and be set to a sane value.

-- 
With Best Regards,
Andy Shevchenko

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