Re: [PATCH v1 6/8] gpio: acpi: Explain how to get GPIO descriptors in ACPI case

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

On Wed, 2017-03-29 at 18:04 +0300, Andy Shevchenko wrote:
> On Wed, 2017-03-29 at 00:12 -0700, Dmitry Torokhov wrote:
> > On Tue, Mar 28, 2017 at 07:39:23PM +0300, Andy Shevchenko wrote:
> > > On Thu, 2017-03-23 at 13:28 -0700, Dmitry Torokhov wrote:
> > > > On Thu, Mar 23, 2017 at 09:46:16PM +0200, Andy Shevchenko wrote:
> > > 
> > > 
> > > > > +Using the _CRS fallback
> > > > > +-----------------------
> > > > > +
> > > > > +If a device does not have _DSD or the driver does not create
> > > > > ACPI
> > > > > GPIO
> > > > > +mapping, the Linux GPIO framework refuses to return any
> > > > > GPIOs.
> > > > > This
> > > > > is
> > > > > +because the driver does not know what it actually gets. For
> > > > > example
> > > > > if we
> > > > > +have a device like below:
> > > > > +
> > > > > +  Device (BTH)
> > > > > +  {
> > > > > +      Name (_HID, ...)
> > > > > +
> > > > > +      Name (_CRS, ResourceTemplate () {
> > > > > +          GpioIo (Exclusive, PullNone, 0, 0,
> > > > > IoRestrictionNone,
> > > > > +                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
> > > > > +          GpioIo (Exclusive, PullNone, 0, 0,
> > > > > IoRestrictionNone,
> > > > > +                  "\\_SB.GPO0", 0, ResourceConsumer) {27}
> > > > > +      })
> > > > > +  }
> > > > > +
> > > > > +The driver might expect to get the right GPIO when it does:
> > > > > +
> > > > > +  desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
> > > > > +
> > > > > +but since there is no way to know the mapping between "reset"
> > > > > and
> > > > > +the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
> > > > > +
> > > > > +The driver author can solve this by passing the mapping
> > > > > explictly
> > > > > +(the recommended way and documented in the above chapter).
> > > > 
> > > > If the driver is not platform specific, then it would have no
> > > > idea
> > > > about
> > > > mapping between _CRS GPIOs and names. All such stuff should be
> > > > hidden
> > > > in
> > > > platform glue (i.e drivers/platform/x86/platform_crap.c).
> > > 
> > > It might be interpreted that all platform data from all the
> > > drivers
> > > should gone. While ideal case should be like this and I totally
> > > agree
> > > with you, we are living in non-ideal world, that's why we used to
> > > and
> > > continue using some ID-based quirks (PCI enumeration, I2C
> > > enumeration,
> > > ACPI enumeration, SPI enumeration, UART enumeration, an so on, so
> > > on).
> > > 
> > > Moreover ACPI comes into ARM(64) world which might have its own
> > > troubles
> > > with generating correct tables and we might end up with quirks
> > > there.
> > 
> > *gasp* I thought ACPI was the magic that would fix all issues with
> > cure
> > embedded hacks.
> 
> In which version of the spec? I think ACPI r6.2 (anticipating soon)
> would have solved a lot of issues regarding GPIO and pin
> configuration.
> 
> I also was and is thinking that ACPI has its own strong sides.
> 
> > > 
> > > So, I disagree that here is possible to hide like you said "all
> > > such
> > > stuff in ...platform_crap.c".
> > 
> > Well, Hans already posted such patch for select x86 platforms with
> > Silead touchscreens. I am sure these platforms have more warts that
> > could be added to the same file in platform/x86/...
> 
> So, do we agree on the following paragraph will be added to this
> documentation?
> 
> "GPIO ACPI mapping tables should not contaminate drivers that are not
> knowing about which exact device they are servicing on. It implies
> that
> GPIO ACPI mapping tables are hardly linked to ACPI ID of the device in
> question and their location is determined solely by location of the
> ACPI
> ID table."
> 
> > > > > +
> > > > > +Getting GPIO descriptor
> > > > > +-----------------------
> > > > > +
> > > > > +There are two main approaches to get GPIO resource from ACPI:
> > > > > +	desc = gpiod_get(dev, connection_id, flags);
> > > > > +	desc = gpiod_get_index(dev, connection_id, index,
> > > > > flags);
> > > > > +
> > > > > +We may consider two different cases here, i.e. when
> > > > > connection
> > > > > ID
> > > > > is
> > > > > +provided and otherwise.
> > > > > +
> > > > > +Case 1:
> > > > > +	desc = gpiod_get(dev, "non-null-connection-id",
> > > > > flags);
> > > > > +	desc = gpiod_get_index(dev, "non-null-connection-id",
> > > > > index, flags);
> > > > > +
> > > > > +Case 2:
> > > > > +	desc = gpiod_get(dev, NULL, flags);
> > > > > +	desc = gpiod_get_index(dev, NULL, index, flags);
> > > > > +
> > > > > +Case 1 assumes that corresponding ACPI device description
> > > > > must
> > > > > have
> > > > > +defined device properties and will prevent to getting any
> > > > > GPIO
> > > > > resources
> > > > > +otherwise.
> > > > > +
> > > > > +Case 2 explicitly tells GPIO core to look for resources in
> > > > > _CRS.
> > > > > +
> > > > > +Be aware that gpiod_get_index() in cases 1 and 2, assuming
> > > > > that
> > > > > there
> > > > > +are two versions of ACPI device description provided and no
> > > > > mapping
> > > > > is
> > > > > +present in the driver, will return different resources.
> > > > > That's
> > > > > why
> > > > > a
> > > > > +certain driver has to handle them carefully as explained in
> > > > > previous
> > > > > +chapter.
> > > > 
> > > > I think that this wording is too x86-centric. We are talking
> > > > about
> > > > consumers of GPIOs here (i.e. drivers), which need unified
> > > > behavior
> > > > between ACPI, DT, and static board properties, they do not
> > > > really
> > > > care
> > > > about _CRS or _DSD.
> > > 
> > > If the certain driver cares about ACPI enumerated devices it might
> > > care
> > > about supporting it disregarding on how new firmware is used
> > > (supporting
> > > _DSD or not).
> > 
> > The drivers might care about ACPI enumerations, but they do not care
> > about warts of particular platform that chose to implement their
> > ACPI
> > tables with missing or invalid data. I say that such knowledge
> > should
> > not go into generic driver, but rather some other entity that woudl
> > fix
> > up whatever wrong the platform did. It could be an ACPI table
> > override,
> > or block of code in platform/x86/..., DT overlay, it does not really
> > matter as long as we do not litter drivers with hacks for random
> > boxes.
> > Yes, we used to do that (DMI tables, etc), because there was no
> > better
> > alternative. Now that we have generic device properties, we have
> > better
> > ways of addressing these issues.
> 
> See above.
> 
> Otherwise I'm reading something like this:
> "If we have platform driverX.c which has DT/platform and ACPI
> enumeration, we must split ACPI part out, duplicate a lot of code and
> use platform driver as a library."
> 
> Is that what you mean?
> 
> P.S. This all _CRS fallback shouldn't be allowed in the first place.
> Now
> I'm trying to sort this crap out to nicely work with _DSD enabled
> firmwares without breaking devices with *old* firmwares. I see no
> other/better way to do such things and I'm open to any _example_ how
> it
> can be done differently.

So, Dmitry, do you have anything to share?

I'm about to submit new version and since we at -rc5, I would really go
with it.

-- 
Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Intel Finland Oy
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