linux-kernel - Re: [PATCH v6 3/3] gpio: TS-5500 GPIO support

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Date:	Fri, 18 May 2012 13:59:25 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:	x86@...nel.org,
	Jerome Oufella <jerome.oufella@...oirfairelinux.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	lm-sensors@...sensors.org,
	Guenter Roeck <guenter.roeck@...csson.com>,
	Jean Delvare <khali@...ux-fr.org>,
	Linus Walleij <linus.walleij@...ricsson.com>
Subject: Re: [PATCH v6 3/3] gpio: TS-5500 GPIO support

On Fri, May 18, 2012 at 8:37 AM, Vivien Didelot
<vivien.didelot@...oirfairelinux.com> wrote:
> Hi Grant,
>
> On Thu, 2012-05-17 at 16:59 -0600, Grant Likely wrote:
>> On Thu, May 17, 2012 at 3:40 PM, Vivien Didelot
>> <vivien.didelot@...oirfairelinux.com> wrote:
>> > On Thu, 2012-05-17 at 15:06 -0600, Grant Likely wrote:
>> >> >  arch/x86/include/asm/ts5500.h |   62 ++++++++
>> >>
>> >> Why the separate header file?  What will use these defines?  I
>> >> normally expect driver-specific defines to be in the driver .c file
>> >> directly; particularly for things like gpio drivers which should be a
>> >> generic interface that doesn't need to export symbols.
>> >
>> > Should an intermediate driver directly use values for GPIOs instead of
>> > these symbols? For example, how should a temperature sensor plugged on
>> > this platform refer to inputs and outputs?
>>
>> Tell me more about this platform.  Where does the data about
>> connections come from?  Is it a purpose-built embedded system?
>
> This is a generic purpose platform, the GPIO bus is exposed on an
> external DB25 connector. End-users can use it however they like.
>
>> Is the
>> GPIO controller described in ACPI? (probably not since GPIOs were only
>> added to ACPI in v5)  Does the end-user attach her own hardware to the
>> board like the temperature sensor you describe?  If so, is that
>> hardware driven by kernel drivers or user-space drivers?
>
> Both in fact. For instance, we are connecting an SHT15
> humidity/temperature sensor, which already has support in the kernel.
>
>>
>> For userspace drivers you can get information about the GPIO number
>> assignments from /sys, but it isn't well documented and can probably
>> be improved.
>>
>> If it is kernelspace, then you really need a way to add data about the
>> platform to the kernel at runtime.  Having it statically compiled in
>> isn't a very good solution.  I would recommend injecting configuration
>> data into the kernel from userspace.  You could invent something, but
>> that wouldn't be very portable.  Xilinx has done some work on this
>> using Flattened Device Tree and the firmware loading infrastructure.
>> The kernel requests a .dtb (device tree blob) from userspace and uses
>> that to configure devices.  That may do the job for you.  GPIO and
>> platform device infrastructure already have FDT support which will
>> help you here.  I expect it could be done with an ACPI fragment too,
>> but I just don't know of anybody having done any work in this area.
>>
>> That probably isn't the answer you want though since I assume you just
>> need to get something that works rather than investing a whole bunch
>> of time on generic infrastructure.
>
> Exactly.
>
>>  What I would recommend is for your
>> platform setup code to use a notifier to wait for the
>> BUS_NOTIFY_BOUND_DRIVER event and then register the temperature sensor
>> with the correct gpio number at that time (because once you have a
>> reference to the gpio controller you can calculate the assigned gpio
>> numbers).
>
> Thanks.
>
> I've been working on pushing this code mainline for a while. To
> summarize, for you to accept this code, you'd prefer me to move every
> symbol into the driver itself (in addition to addressing your and Joe's
> other requests), and then we're good?

Either that or have a *really good* argument why it should be
exposed/exported.  It is already a tough-fought battle to move away
from driver-specific hacks and custom platform code, so I don't want
to be adding yet more if at all avoided.

g.
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