Re: [PATCH] gpio: add gpio_of_helper

[Alexandre Courbot <gnurou@xxxxxxxxx>]

On Thu, Oct 23, 2014 at 3:23 PM, Jiří Prchal <jiri.prchal@xxxxxxxxxxx> wrote:
>
>
> Dne 23.10.2014 v 07:08 Alexandre Courbot napsal(a):
>
>> On Wed, Oct 22, 2014 at 6:58 PM, Pantelis Antoniou
>> <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> Hi Alexandre,
>>>
>>>> On Oct 22, 2014, at 12:41 PM, Alexandre Courbot <gnurou@xxxxxxxxx>
>>>> wrote:
>>>>
>>>> On Wed, Oct 22, 2014 at 6:24 PM, Pantelis Antoniou
>>>> <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> Hi Alexandre,
>>>>>
>>>>>> On Oct 22, 2014, at 12:18 PM, Alexandre Courbot <gnurou@xxxxxxxxx>
>>>>>> wrote:
>>>>>>
>>>>>> On Wed, Oct 22, 2014 at 5:26 PM, Jiri Prchal <jiri.prchal@xxxxxxxxxxx>
>>>>>> wrote:
>>>>>>>
>>>>>>> This patch adds new driver "gpio-of-helper", witch has possibility to
>>>>>>> export
>>>>>>> gpios defined in dt. It exports them in defined name under
>>>>>>> /sysfs/class/gpio/name.
>>>>>>> It's rebased from Pantelis Antoniou patch to new kernel.
>>>>>>> Usage example:
>>>>>>>        gpio {
>>>>>>>                compatible = "gpio-of-helper";
>>>>>>>                status = "okay";
>>>>>>>                pinctrl-names = "default";
>>>>>>>                pinctrl-0 = <&pinctrl_gpio>;
>>>>>>>
>>>>>>>                gsm_on {
>>>>>>>                        gpio-name = "gsm_on";
>>>>>>>                        gpio = <&pioB 13 GPIO_ACTIVE_HIGH>;
>>>>>>>                        output;
>>>>>>>                        init-low;
>>>>>>>                };
>>>>>>>        };
>>>>>>>
>>>>>>> This patch needs Alexey Ignatov [PATCH] gpiolib: allow exporting
>>>>>>> gpios with
>>>>>>> custom names.
>>>>>>
>>>>>>
>>>>>> We will need to see whether the pre-requisite patch can get merged
>>>>>> first, but there are a couple of things that are wrong with your patch
>>>>>> anyway:
>>>>>>
>>>>>> - it is missing a bindings documentation
>>>>>> - it is using the legacy integer GPIOs instead of the descriptor
>>>>>> interface (see include/linux/gpio/consumer.h). Since this is DT-based,
>>>>>> there is absolutely no reason to not use the descriptors interface.
>>>>>> - it seems quite long for what it needs to do
>>>>>> - the MODULE_AUTHOR has not signed-off this patch (?)
>>>>>>
>>>>>> But what makes me nervous is that this encourages more usage of the
>>>>>> sysfs interface, an in a way that is potentially harmful.
>>>>>>
>>>>>> Also, I don't know if the DT people will be happy with this idea.
>>>>>> Since this concerns DT, please also add the devicetree list and get a
>>>>>> Acked-by for the bindings you want to push by a DT maintainer.
>>>>>
>>>>>
>>>>> Since I’m the original perpetrator, let me put a few words here for
>>>>> posterity.
>>>>>
>>>>> This patch was meant as a quick and dirty method for doing the
>>>>> automatic export
>>>>> and pinmux configuration from DT on a 3.8 kernel. The kernel APIs have
>>>>> moved on
>>>>> since. It wasn’t meant to be submitted for mainline right as it is.
>>>>>
>>>>> Unfortunately (or fortunately for many people) it does what a lot of
>>>>> people need
>>>>> i.e. configures a board with heavily pinmuxed GPIO right at boot time.
>>>>>
>>>>> I’m open at having a small discussion about how to do what the users of
>>>>> this patch
>>>>> do ‘right’.
>>>>
>>>>
>>>> Sure, although the discussion might turn out to not be that "small". :)
>>>>
>>>
>>> Heh ;)
>>>
>>>> Pinmux configuration sounds like a job for pinmux more than GPIO, and
>>>> exporting potentially many GPIOs to user-space sounds like a work for
>>>> a proper driver.
>>>>
>>>
>>> I’m afraid that’s not the case. A great many users do not require
>>> anything
>>> more than setting a pinmux and the GPIO configuration (input/output).
>>> They can then do low speed I/O using the sysfs interface, without having
>>> to
>>> use any complex APIs (shell works just fine).
>>>
>>> Think of stuff like controlling a sprinkler valve, or something like a
>>> mechanical
>>> door detection open state.
>>
>>
>> That sounds like any kind of arbitrary device that can be temporarily
>> connected to a set of GPIOs that will be bit-banged. Is my
>> interpretation correct?
>
>
> Not only temporarily. For example on/off switch of some hw on board like gsm
> modem.

... which should be handled in-kernel.

> Or some optocoupler isoleted in/outs for general use from userspace.

... which should thus be configured from userspace.

>>
>>
>> In that case, I seriously doubt that this should be part of the DT.
>> Right now the DT is part of the firmware, and an immutable description
>> of the hardware layout of a board - definitely such devices do not fit
>
> This is our case of gpio. It's like LEDs, they are hw layout of board,
> but they are in/out instead of LEDs are only output.
>>
>> into that definition. This might change once the Device Tree Overlays
>> are merged, but we are not there yet. If the DT maintainers say this
>> is a valid use-case for overlays, then I will reconsider my position,
>> but in any case it really looks like this could/should be done from
>> user-space.
>
> I don't think so as I explained - HW layout of boerd.

If it is a static hardware layout that makes the case even clearer -
these devices should have a proper kernel driver that configures these
GPIOs, and exports a proper interface to userspace, not just raw
GPIOs.

> In other way I'm looking forward for DT overlays.
>>
>>
>> You have almost all the necessary pieces: you can export, configure
>> and manipulate any GPIO from the sysfs interface. The only thing you
>> would be missing is a way to give a name to a GPIO, something that can
>> easily be fixed.
>
> But isn't it nice to have already exported gpios which are layouted out
> from board. All other unexported gpios aren't connected to anywhere on
> board. So user don't have to export or unexport anything.
>>
>>
>> Since the devices you want to configure that way are typically
>> removable or usage-specific, why would you want to store that
>> information all the way up into the firmware? A commonly accepted
>> wisdom is that what can be done in user-space should be done in
>> user-space, and it really looks like this applies here.
>
> They are NOT removable.
>>
>>
>> Say you buy a Jetson TK1 to control that sprinkler valve (a good use
>> for all these GPU cores!). The mainline DT has no knowledge about the
>> valve, so using your proposed way you will have to re-build and flash
>> a custom device tree just to be able to use your sprinkler. If you
>> decide to assign your board to something else, you will need another
>> DT.
>
> But if you buy more "real world ready" board with for example 8 power
> outputs for sprinkler valves, why has user think about what gpio to
> export, if there could be valve0 - 7 ready. If he will use only valve0
> does not mater, all unused gpio can't be used for anything else since
> they has layout for valves.

The problem is indeed different.

If your sprinkler valves are wired on your board and the lines cannot
be used for anything else, then what you need is a driver for the
valve that will acquire the GPIOs and export its own userspace
interface under /sys/.

You device tree must reflect the layout of the board ; in this case
"here is a valve that uses this GPIO active-low". Saying "this GPIO is
output active-low and named valve0" is not a good description of your
hardware.

>>
>>
>> Instead, have a shell script or a systemd unit tmpfile that will
>> perform the correct setup at boot time. Then you can easily switch
>> usages from user-space (systemctl stop sprinkler && systemctl start
>> doordetect). This is much, much more flexible. At least until the
>> Device Tree Overlays are merged, but even then this seems overkill.
>
> I seriously think about it, but I think if LEDs are in DT, GPIOs should
> be there too. In other way, I don't like exporting without given name.

LEDs are actually a perfect example of what you should do. They are a
simple device often plugged to a GPIO that controls them. We could
very well do what you propose: export GPIOs named "ledXX" and call it
a day. Instead we have a driver that exports a proper device node in
/sys with the operations that are relevant to a LED.

Once your GPIOs are assigned to a given function, they are not
"general purpose" anymore. Therefore you should expose the right
abstraction to your users, even if that means writing some more code.
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