Re: [PATCH V4 2/2] gpio: inverter: document the inverter bindings

[Harish Jenny K N <harish_kandiga@xxxxxxxxxx>]

Hi Geert,


On 07/10/19 1:48 PM, Geert Uytterhoeven wrote:
> Hi Eugeniu,
>
> On Sat, Oct 5, 2019 at 3:08 PM Eugeniu Rosca <roscaeugeniu@xxxxxxxxx> wrote:
>> On Fri, Sep 27, 2019 at 11:07:20AM +0200, Geert Uytterhoeven wrote:
>>> My standard reply would be: describe the device connected to the GPIO(s)
>>> in DT.  The GPIO line polarities are specified in the device's "gpios"
>>> properties.
>>> Next step would be to use the device from Linux.  For that to work, you
>>> need a dedicated driver (for the complex case), or something generic
>>> (for the simple case).
>>> The latter is not unlike e.g. spidev.  Once you have a generic driver,
>>> you can use "driver_override" in sysfs to bind the generic driver to
>>> your device.  See e.g. commit 5039563e7c25eccd ("spi: Add
>>> driver_override SPI device attribute").
>> We have passed your suggestions along. Many thanks.
>>
>>> Currently we don't have a "generic" driver for GPIOs. We do have the
>>> GPIO chardev interface, which exports a full gpio_chip.
>>> It indeed looks like this "gpio-inverter" could be used as a generic
>>> driver.  But it is limited to GPIOs that are inverted, which rules out
>>> some use cases.
>>>
>>> So what about making it more generic, and dropping the "inverter" from
>>> its name, and the "inverted" from the "inverted-gpios" property? After
>>> all the inversion can be specified by the polarity of the GPIO cells in
>>> the "gpios" property, and the GPIO core will take care of it[*]?
>>> Which boils down to adding a simple DT interface to my gpio-aggregator
>>> ("[PATCH/RFC v2 0/5] gpio: Add GPIO Aggregator Driver",
>>>  https://lore.kernel.org/lkml/20190911143858.13024-1-geert+renesas@xxxxxxxxx/).
>>> And now I have realized[*], we probably no longer need the GPIO
>>> Forwarder Helper, as there is no need to add inversion on top.
>> After having a look at the gpio aggregator (and giving it a try on
>> R-Car3 H3ULCB), here is how I interpret the above comment:
>>
>> If there is still a compelling reason for having gpio-inverter, then it
>> probably makes sense to strip it from its "inverter" function (hence,
>> transforming it into some kind of "repeater") on the basis that the
>> inverting function is more of a collateral/secondary feature, rather
>> than its primary one. Just like in the case of gpio aggregator, the
>> primary function of gpio inverter is to accept a bunch of GPIO lines and
>> to expose those via a dedicated gpiochip. I hope this is a proper
>> summary of the first point in your comment. In any case, this is the
>> understanding I get based on my experiments with both drivers.
> Yes, the inverter is basically a "repeater" (or "aggregator", when it has
> multiple GPIOs connected), hardcoded to invert.
>
>> What I also infer is that, assuming gpio-inverter will stay (potentially
>> renamed and stripped of its non-essential inverting function), the gpio
>> aggregator will need to keep its Forwarder Helper (supposed to act as a
>> common foundation for both drivers).
> What I meant is that if the inverter and aggregator would be combinoed
> into a single driver, there would no longer be a need[*] for a separate
> helper, and it could be incorporated into the single driver.
>
> [*] The individual helper functions may still be useful for some other
>      driver, though.


Agree.


>> The second point which I extract from your comment is that the "gpio
>> aggregator" could alternatively acquire the role of "gpio-inverter"
>> (hence superseding it) by adding a "simple DT interface". I actually
>> tend to like this proposal, since (as said above) both drivers are
>> essentially doing the same thing, i.e. they cluster a number of gpio
>> lines and expose this cluster as a new gpiochip (keeping the
>> reserved/used gpio lines on hold). That looks like a huge overlap in
>> the functionalities of the two drivers.
> Yes, both drivers are very similar.  The difference lies in how they
> acquire the list of GPIO descriptors.

Yes. In fact my V2 version of the patch tried to implement the same role as repeater/forwarder albeit with a different naming/intention.

Linus Walleij mentioned that using GPIO_ACTIVE_LOW just to get free inversion inside GPIOLIB was not OK really and this is a hardware description problem and totally different from the implementation problem inside the driver.

Hence we changed the logic to inverter consumer driver doing inversion inside get and set functions.

>
>> The only difference which I see is that "gpio-inverter" is getting its
>> input from DT and generates the gpiochips at probe time, while
>> "gpio aggregator" is getting its input from sysfs and generates the
>> gpiochips at runtime, post-probe.
> Exactly.
>
> For my virtualization use case, I need to create the list of GPIO
> descriptors at run-time, hence the sysfs interface. This is
> polarity-agnostic (i.e. the end user needs to care about polarity).
>
> For Harish use case, he needs to describe the list from DT, with
> polarity inverted, which can be done by specifying the GPIO_ACTIVE_LOW
> flag in the node's"gpios" property.
>
> For your use case, you want to describe the list in DT, with line-names,
> and polarity specified.
>
>> So, assuming no objections from Harish and other reviewers, I would be
>> very happy to review and test the DT-based gpio inversion functionality
>> as part of gpio aggregator. Thanks!


I tested your aggregator driver with the below minor changes in gpio-aggregator (combined with some minor changes in GPIO forwarder) to get devicetree support.


195,196d194
<     int index = 0;
<     int count;
278,295d275
<     count = gpiod_count(dev, NULL);
<     if (count > 0) {
<         while (index < count) {
<             desc = devm_gpiod_get_index(dev, NULL, index, GPIOD_ASIS);
<
<             if (desc == ERR_PTR(-ENOENT))
<                 return -EPROBE_DEFER;
<
<             if (IS_ERR(desc))
<                 return PTR_ERR(desc);
<
<             error = add_gpio(dev, &descs, &n, desc);
<             if (error)
<                 return error;
<             index++;
<         }
<     }
<
316,319d295
< static const struct of_device_id gpio_aggregator_match[] = {
<     { .compatible =    "gpio-aggregator", }, { },
< };
<
326d301
<         .of_match_table = of_match_ptr(gpio_aggregator_match),


This does work and achieve our aim of inverter driver.

Hence no objection from my side to merge the drivers. Please let me know if I need to send you a patch on top of your aggregator patch.

Hoping to get some credits for my work of 5 months effort ! ;)


Best Regards,

Harish Jenny K N