Re: [PATCH 0/2] gpio: Allow userspace export from DT

["Jiří Prchal" <jiri.prchal@aksignal.cz>]

On 6.5.2015 15:25, Johan Hovold wrote:
> On Wed, May 06, 2015 at 01:57:08PM +0100, Russell King - ARM Linux wrote:
>> On Wed, May 06, 2015 at 02:43:54PM +0200, Johan Hovold wrote:
>>> On Wed, May 06, 2015 at 12:24:50PM +0100, Russell King - ARM Linux wrote:
>>>> On Mon, May 04, 2015 at 10:49:25AM +0200, Johan Hovold wrote:
>>>>> Firmware should describe pin directionality and function, and undefined
>>>>> pins should never be allowed to be accessed from userspace.
>>>>
>>>> No, that's totally wrong if you consider one of the most common use
>>>> cases out there...
>>>>
>>>> Think about something like a Raspberry Pi, where you have a header with
>>>> GPIOs on it, which can be used for multiple different purposes (and are
>>>> even multiplexed with some on-SoC functions.)
>>>>
>>>> "Firmware" can't know about all possible configurations of those IO pins.
>>>>
>>>> That's why Raspberry Pi uses a userspace helper and programs stuff up
>>>> appropriately for the users application.
>>>
>>> I'm not familiar with exactly how the RPi handles its pin muxing, but
>>> even if it requires userspace interaction that should not prevent having
>>> firmware describe the pins.
>>
>> How it handles this is... it doesn't.  Userspace does it.
>>
>> There is *no* "firmware" on these devices.  The only thing you have is a
>> boot loader and a DT blob, and people will hate you if you tell them that
>> they have to change the DT blob and then reboot their systems to change
>> the functionality of a GPIO pin.
>>
>> It's also entirely reasonable to assume that people are going to want to
>> change the configuration at runtime, given their diverse range of
>> applications.
>
> DT can be changed at run-time using overlays.
>
> This way the RPi users could define their i2c-clients, or whatever buses
> they choose to expose on those pins as well.
>
>>> In general, if the hardware configuration is static we use device trees.
>>>
>>> If the configuration is dynamic we use device-tree overlays; either
>>> loaded manually or by some service (e.g. Beaglebone capes). Perhaps this
>>> could be handled by the RPi helper.
>>
>> Yes, that _could_ work, but I bet asking millions of people to change
>> what they're doing is going to be extremely difficult.  Remember the
>> golden rule of the kernel: thou shalt not break userspace.
>>
>> So, there is _no_ possibility of removing this GPIO exposure to userspace
>> because we _know_ that it will break people.
>>
>> If you think differently on the "thou shalt not break userspace" please
>> don't discuss it with me, I'm not interested.  Linus isn't interested
>> either, and if you try and discuss it with him, he'll tell you to get
>> out of kernel programming. :)
>>
>> This is a commonly used API.  You can't change it.
>
> I've never suggested that we remove the current API, but *only* exposing
> a totally non-restrictive interface for systems not used by school
> children and hardware hackers is not sane.
>
> You also cut out the part in my reply about continuing to allow
> unrestricted access for such cases. That could still continue to be the
> default (e.g. when there are no pin function names defined in DT).
>
> And if those people ever update their DTs they could benefit from the
> deterministic pin naming without loosing any flexibility too.

Just to not let Johan alone:
I totally agree with Johan and looking for this export for at least 2 years.
Jiri

>
> Johan
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