[PATCH v4 3/4] gpio: Add find GPIO base in increasing order

[Alexandre Courbot]

On Mon, Dec 15, 2014 at 1:01 PM, Zhou Wang <wangzhou.bry at gmail.com> wrote:
> On 2014年12月10日 16:51, Alexandre Courbot wrote:
>>
>> On Fri, Dec 5, 2014 at 12:38 PM, Zhou Wang <wangzhou.bry at gmail.com> wrote:
>>>
>>> In function gpiochip_find_base, base number of a GPIO controller
>>> is found in decreasing order. ARCH_NR_GPIOS is used to define from
>>> which number we begin to search for base number of a GPIO controller.
>>>
>>> In fact, ARCH_NR_GPIOS brings us some multiplatform problems, like:
>>> http://www.spinics.net/lists/devicetree/msg60433.html
>>>
>>> This patch adds the support to find base number of a GPIO controller
>>> in increasing order. It will assign base number from 0.
>>> A new dts property called gpio-number-forward must be add to the related
>>> GPIO dts nodes if you want it works well.
>>
>>
>> Global GPIO numbers are a Linux-only concept, so your property should
>> be named linux,gpio-number-forward.
>>
>> But even that way I don't think I like this idea. This just adds some
>> more mess to how the GPIO number space is constructed, and it is
>> already quite messy as it is. You have no guarantee over the probe
>> order of your GPIO controllers, so they may very well be probed in a
>> different order and end up with different base numbers anytime.
>>
>> Not that this is your fault - the number namespace is broken by design
>> and I don't think there is a way to fix it. The long-term solution is
>> to stop using it by switching to the gpiod interface.
>>
>> First a few questions to understand why you need your GPIOs numbered
>> this way, and if you need it at all:
>> - Can't you use the gpiod interface instead so you don't need to rely
>> on GPIO numbers?
>
>
> Hi Alexandre,
>
> Sorry for late. Could you give me more clue about the gpiod interface? Don't
> we also call gpio_request() which uses GPIO number to request a
> GPIO?

See Documentation/gpio/consumer.txt and Documentation/gpio/board.txt.
You can obtain a GPIO descriptor without using a number by calling
gpiod_get(). Prior to that, individual GPIOs need to be bound to
devices and functions using either DT (preferred) or the platform
interface. The old integer-based GPIO interface is considered
deprecated, although still widely used. But new code should rely
exclusively on gpiod, and we encourage people to convert existing code
to it too.

>
>> - Do you plan to use the sysfs interface? If so then we are screwed,
>> because there is no way to use it without using global GPIO numbers.
>
>
> I am now enabling GPIO in Hip04-d01. Maybe, I can just use
> the default ARCH_NR_GPIOS, then users can manage GPIO through sysfs.
> However if so, GPIO 0~127 will be mapped to GPIO 384~511.

Yeah, I know that's not ideal. As a workaround, users can maybe
identify the right gpiochip by parsing /sys/class/gpio/gpiochip* and
comparing the "label" node. Once the right chip is found, its "base"
entry gives the base GPIO number which can be used to export the
desired GPIO.

We are also taking steps to come with a better sysfs interface. I will
keep you in the loop.

>
>> This is something we should/will fix with named exported GPIOs, but we
>> are not here yet.
>>
>> As to how we can solve your problem: if you must use GPIO integers
>> (because you need to use the sysfs interface for instance), and need
>> them affected consistently, the only way I can think of is to
>> introduce a "linux,gpio-base" property that will set gpiochip->base to
>> a fixed number. It still kind of sucks, but at least it will enforce
>
>
> Thanks for your suggestion. But setting "linux,gpio-base" will bring
> gpio base implementation specific, and in face there is no place to gain
> this gpio base info, not appropriate both in gpio arch code and dwapb
> code.

Yeah, besides this property did not receive a warm reception. So my
suggestion for now is to workaround the issue using the technique
described above, until we come with a better sysfs interface that does
not rely on GPIO numbers. Sorry for that inconvenience.