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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Harish,

On Tue, Nov 12, 2019 at 12:52 PM Harish Jenny K N
<harish_kandiga@mentor.com> wrote:
> On 11/10/19 10:05 AM, Harish Jenny K N wrote:
> > On 07/10/19 1:48 PM, Geert Uytterhoeven wrote:
> >> On Sat, Oct 5, 2019 at 3:08 PM Eugeniu Rosca <roscaeugeniu@gmail.com> 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@glider.be/).
> >>>> 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
>
>
> Is any attempt being made for the newer version of the aggregator/inverter driver ?

It's on my list, and I hope to tackle it soon (later this week, or next week).
Thanks for your patience!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds