linux-kernel - Re: [PATCH v2 2/4] gpio: Allow PC/104 devices on X86

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Date:	Sat, 4 Jun 2016 07:12:21 -0400
From:	William Breathitt Gray <vilhelm.gray@...il.com>
To:	Pavel Machek <pavel@....cz>
Cc:	gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
	x86@...nel.org, linux-next@...r.kernel.org,
	linux-gpio@...r.kernel.org, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
	Guenter Roeck <linux@...ck-us.net>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v2 2/4] gpio: Allow PC/104 devices on X86_64

On Sat, Jun 04, 2016 at 09:14:08AM +0200, Pavel Machek wrote:
>On Fri 2016-06-03 17:12:44, William Breathitt Gray wrote:
>> On Fri, Jun 03, 2016 at 10:57:03PM +0200, Pavel Machek wrote:
>> >Should we do "depends on PC104" here, because that is what it really
>> >means, and have PC104 enabled when ISA_BUS_API is enabled or something
>> >like that?
>> 
>> Since the functionality remains the same, I'm a bit indifferent to that
>> change; as long as the driver builds for systems in which it's intended
>> to be used, I'm satisfied.
>> 
>> Differentiating between PC/104 and ISA may be a pointless endeavor
>> though since both buses appear the same to software. But if it is better
>> to differentiate between devices as such, then I see little harm in
>> adding a PC104 Kconfig option which follows the ISA_BUS_API Kconfig
>> option.
>
>Well, they are same to the software, but not at the hardware. If I
>have a development board that has PC104 (but not isa), I'd like to see
>prompts for PC104 extensions, not for isa. If PC105 comes out, still
>ISA compatible, I will want to see prompts for PC104 boards or PC105
>boards, but not neccessarily both...

I think I see the merit of a prompt for PC104 devices. I've encountered
a use case recently which I'm curious about in this scenario. Given the
compatibility with ISA, manufacturers may occasionally develop variants
of existing ISA devices by duplicating the firmware on a PC/104 form
factor.

I'm working on an IIO DAC driver for the Measurement Computing CIO-DAC
family (CIO-DAC08, CIO-DAC16, and PC104-DAC06); while not a GPIO driver,
I believe it can serve as a decent example. Interestingly, while the
CIO-DAC08 and CIO-DAC16 are true ISA devices, the PC104-DAC06 is a
PC/104 variant compatible with the others in the family. The IIO DAC
driver works just as well with the PC104-DAC06, as it does with the true
ISA devices in the family.

What would the Kconfig depends line look in this scenario? I imagine
simply "depends on PC104" would be inappropriate since there are a
number of true ISA devices supported by the driver, but "depends on
ISA_BUS_API || PC104" seems somewhat redundant when the PC104 Kconfig
option implies ISA_BUS_API. This situation isn't that much of an issue
overall, but I anticipate encountering it occassionally as I develop
future PC/104 drivers.

William Breathitt Gray