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

[Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>]

On Tue, Apr 04, 2017 at 07:11:17PM +0300, Andy Shevchenko wrote:
> 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.

Sorry, that was a dig at someone else ;)

> > 
> > > > 
> > > > 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."

I completely agree with the 1st sentence and disagree about the 2nd. The
ACPI ID usually describes a chip, much like DT compatible does, and
rarely is platform specific. For example, we have i2c-hid binding to
ACPI0C50 or PNP0C50. These are standard IDs shared between many
platforms. Same goes for various Silead IDs; Chromebooks use ATML0000
and ATML0001 for Atmel touch controllers, ELANXXXX for Elan controllers,
etc. All of them might have different connections, depending on platform
(i.e. on given platform firmware might be responsible for powering up
and resetting controller while on another it is done by the kernel). The
driver that is not platform specific should ideally not have
platform-specific knowledge in it, but instead get configuration from
elsewhere (device tree, acpi, board code, or platform driver augmenting
the former 3 sources, like we have with silead_dmi.c).

> > 
> > > > > > +
> > > > > > +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."

No. You need to split the part that augments incomplete ACPI data, and
move it somewhere (drivers/platform/x86/<platform>-crap.c; the driver
stays the same: a driver that is useful across multiple platforms.

> > 
> > Is that what you mean?
> > 
> > P.S. This all _CRS fallback shouldn't be allowed in the first place.

It does work in many cases. By disallowing it completely you force much
more platform stuff knowledge in the kernel, whereas before you needed
to deal with exceptions.

Thanks.

-- 
Dmitry
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