Message-ID: <20130826151556.GD13942@kw.sim.vm.gnt>
Date:	Mon, 26 Aug 2013 17:15:57 +0200
From:	Simon Guinot <simon.guinot@...uanux.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v3] gpio: add GPIO support for F71882FG and F71889F

On Thu, Aug 01, 2013 at 08:52:32AM -0700, Guenter Roeck wrote:
> On 08/01/2013 06:46 AM, Simon Guinot wrote:
> >On Mon, Jul 29, 2013 at 05:59:08PM +0200, Linus Walleij wrote:
> >>On Mon, Jul 22, 2013 at 11:50 AM, Simon Guinot
> >><simon.guinot@...uanux.org> wrote:
> >>
> >>>This patch adds support for the GPIOs found on the Fintek super-I/O
> >>>chips F71882FG and F71889F.
> >>>
> >>>A super-I/O is a legacy I/O controller embedded on x86 motherboards. It
> >>>is used to connect the low-bandwidth devices. Among others functions the
> >>>F71882FG/F71889F provides: a parallel port, two serial ports, a keyboard
> >>>controller, an hardware monitoring controller and some GPIO pins.
> >>>
> >>>Note that this super-I/Os are embedded on some Atom-based LaCie NASes.
> >>>The GPIOs are used to control the LEDs and the hard drive power.
> >>>
> >>>Signed-off-by: Simon Guinot <simon.guinot@...uanux.org>
> >>>---
> >>>Changes since v2:
> >>>- Remove useless NULL setters for driver data.
> >
> >Hi Linus,
> >
> >>
> >>Given the recent discussion with Rafael I want to have an
> >>extended discussion of this patch.
> >>
> >>It is my current understanding that:
> >>
> >>- It is possible to define the whereabouts of the SuperIO
> >>   chips using ACPI
> >
> >Agreed.
> >
> >>- It is possible for developers to influence the source
> >>   AML for the DSDT tables of these systems.
> >
> >I am not sure about that. Let's consider the LaCie x86-based boards.
> >LaCie only adds a few devices on the top of a motherboard provided by
> >an another manufacturer. In turns, this last gets a Super-I/O from an
> >another manufacturer. In my understanding, the Super-I/O manufacturer is
> >responsible for registering the PNP IDs (one per device functionality).
> >
> >LaCie may have enough leverage to obtain some modifications on the ACPI
> >DSDT tables but about the PNP IDs registration, let's say it is less
> >that certain. The problem is that LaCie don't have any contacts with the
> >Super-I/O manufacturer.
> >
> >I have to say that all this process is not as easy as adding a node in
> >a dts file.
> >
> >>- It is the proper thing to do.
> >
> >Yes, it may be.
> >
> >>- So we should atleast support ACPI probing with the
> >>   port-based detection as a final fallback if all else fails.
> >>
> >>Why can I not get something like:
> >>
> >>#include <linux/acpi.h>
> >>(...)
> >>static const struct acpi_device_id gpio_acpi_match[] = {
> >>         { "FOOBAR", 0 },
> >
> >After some checks on my boards, it appears that there is no PNP ID
> >available for the Super-I/O GPIO functionality (or any others). Moreover
> >I think this IDs don't have been registered to Microsoft by Fintech
> >(the super-I/O manufacturer).
> >
> >How do you envisage the follow-up ?
> >
> >>         { }
> >>};
> >>MODULE_DEVICE_TABLE(acpi, gpio_acpi_match);
> >>
> >>static struct platform_driver gpio_driver = {
> >>         (...)
> >>         .driver         = {
> >>                  (...)
> >>                 .acpi_match_table = ACPI_PTR(gpio_acpi_match),
> >>         },
> >>};
> >
> >It seems to me that the ACPI probing is the easiest part. How do you see
> >the ioport probing fallback ?
> >
> >I can only figure out broken solutions:
> >
> >1. From the init function, we could check that the PNP IDs are well
> >    available in the ACPI DSDT tables before registering the platform
> >    driver. If not, we could fall back on the ioport probing method.
> >    I don't know if checking the DSDT tables is even possible. It is at
> >    least weird and it defeats completely the purpos of acpi_match_table.
> >2. In a late initcall, we could check that the driver is well
> >    registered else fall back on the ioport detection.
> >    As GPIOs may be needed early, I don't think this method is suitable.
> >
> >And I have no more ideas...
> >
> 3. Implement Super-IO detection in the the ACPI platform driver.
>    If there is no ACPI device entry for a detected Super-IO chip's sub-function(s),
>    fake it and create the respective platform device(s).
> 
> Just as kludgy as your proposed solutions, but at least it would move Super-IO detection
> to one file and let all Super-IO drivers use the ACPI match table.
> 
> Drawbacks:
> - Only works for x86 (ie it would limit SuperIO drivers to x86). Not sure if that is
>   a real limitation - are SuperIO chips used on other platforms ?
> - It would require us to define fake PNP IDs for the various SuperIO functions.
> - It may fail if a firmware / chip vendor ever adds real PNP IDs for the various
>   sub-functions and those start showing up in ACPI tables (maybe that doesn't matter
>   as much as the drivers would have to be updated anyway to match the real IDs).

As an additional drawback, this solution involves systematic I/O port
probing from the ACPI platform driver, even if the platform don't need
it. This may increase the conflicts risk in comparison with probes from
some end drivers (which are more likely built as modules and not loaded
automatically). Except that, moving all this Super-I/O detection in a
same place would be indeed a great benefit.

Linus, how do you see the follow-up for the gpio-f7188x driver ? Let me
know your advice.

Thanks,

Simon

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