[alsa-devel] [PATCH 2/2] ASoC: max98927: Add reset-gpio support

[Cheng-yi Chiang]

Hi Philipp,

On Thu, Oct 18, 2018 at 1:02 AM Philipp Zabel <p.zabel at pengutronix.de> wrote:
>
> Hi Maxime,
>
> On Fri, 2018-10-12 at 15:46 +0200, Maxime Ripard wrote:
> > On Fri, Oct 12, 2018 at 12:05:16PM +0200, Philipp Zabel wrote:
> [...]
> > > What I would like better would be to let the consumers keep their reset-
> > > gpios bindings, but add an optional hold time override in the DT:
> > >
> > >     c1 {
> > >             reset-gpios = <&gpio0 15 GPIO_ACTIVE_LOW>;
> > >             reset-delays-us = <10000>;
> > >     };
> > >
> > >     c2 {
> > >             reset-gpios = <&gpio0 15 GPIO_ACTIVE_LOW>;
> > >             reset-delays-us = <10000>;
> > >     };
> > >
> > > The reset framework could create a reset_control backed by a gpio
> > > instead of a rcdev. I have an unfinished patch for this, but without the
> > > sharing requirement adding the reset framework abstraction between gpiod
> > > and drivers never seemed really worth it.
> >
> > I don't remember the exact details of our past discussion, but I
> > (still) don't really think that this would work well.
>
> It has been a while :) Thanks for jumping back in.
>
> > I see two main shortcomings with that approach:
> >
> >   - I guess that the main reason you want to do that would be to have
> >     easy DT backward compatibility.
>
> Yes, that is true. The other reason is that I'd like devices to have a
> single binding, regardless of whether somebody decided to put them onto
> a board with shared reset lines.
>
> I'd find it hard to advocate for changing the thankfully common case
> of device-exclusive reset gpios from:
>
>         some-device {
>                 reset-gpios = <&gpiox y>;
>         };
>
> to:
>
>         rstc: reset-controller {
>                 compatible = "gpio-reset";
>                 reset-gpios <&gpiox y>;
>         };
>
>         some-device {
>                 resets = <&rstc 0>;
>         };
>
> even for new bindings.
>
> If the reset framework only supports the latter, and not the former,
> drivers for devices which already have reset-gpios would have to handle
> both reset_control and gpiod functionality, which I think should not be
> necessary.
>
> Note that this is not really an argument against a "gpio-reset"
> controller driver, but an argument for reset-gpios support integrated
> into the reset framework.
> My argument against a "gpio-reset" device node in the DT is only that it
> is basically a virtual device that would only be used to work around
> missing reset-gpios support in the reset framework. Physically, this
> "device" consists of no more than a few PCB traces.
>
> >     Yet, the addition of the
> >     reset-delay-us would break that compatibility, since drivers that
> >     would have been converted now need to have it somehow, but older
> >     DTs wouldn't have it
>
> That is why such a property would have to be optional, and the drivers
> would have to keep providing the reset delay themselves, as they
> currently do when handling GPIOs directly.
> It would only serve to override the driver default in case of additional
> delay requirements due to board specifics (such as delay elements, or
> shared resets).
>
> >   - There's so many variations of the reset-gpios property name that
> >     you wouldn't be able to cover all the cases anyhow, at least
> >     without breaking the compatibility (again).
>
> This is true, and I do not have a solution for this. Especially for
> those cases that don't use gpiod yet.
>
>         of_get_named_gpio(node, "reset-gpio")
>
> is still quite common, but I'd really like on gpiolib for polarity
> support.
>
> > But I also see your point, and you're right that converting everyone
> > to a gpio-reset node will not happen (even though I'd still really
> > like to have that binding).
>
> See above. I'd be a lot less reluctant to support this binding if
> somebody could demonstrate a real gpio controlled reset controller
> device of some kind. And even then I would not want to have to use
> this device just to connect a single GPIO with a single reset input.
>
> > What about having a function that would be called by the consumer to
> > instantiate that reset controller from a GPIO for us, instead of
> > trying to do it automatically? That function could take the property
> > name that holds the GPIO, which would cover the second drawback, and
> > the delay, which would cover the first.
>
> I like this idea.
>
> I'd like to avoid having to fall back from gpiod to of_get_named_gpio if
> possible, so maybe we'd have to extend gpiolib-of.c with an
> of_find_reset_gpio function, as is done for SPI and regulators already.
>
> We probably would have to support delay ranges. I see a lot of
> usleep_range calls between reset GPIO toggles in the drivers.
>

This looks great.

I am not sure what is the plan proceeding from here.
Are you going to implement this feature with the unfinished patch you
mentioned to create reset_control backed by a GPIO?
I am willing to help.
If you have patch I can help testing it.
It will be easier to test the shared reset line on my side since the
board I am working now has this use case.

If you don't have time for this, I can work on it based on your WIP patch.
In that case could you please post the patch ?
I just want to make sure I am on the right direction solving this
without making duplicate efforts..

> > And in order to cover shared GPIO reset lines, we could just look at
> > the one already created by other drivers, and if one has been created,
> > we just give the reference to that one instead of creating it.
> >
> > Does that make sense?
>
> I'm not quite sure how to match an already requested reset control from
> the list given of_node and gpio property name at the moment - this might
> involve exporting gpiod_find from gpiolib, but API wise I think this is
> a sane proposal.
>
> I would not want to add reset controller devices for each of these GPIO
> reset controls, but rather store the gpio_desc reference in the
> reset_control instead of the rcdev reference.

This plan to support shared reset line makes sense.

Thanks a lot!

>
> regards
> Philipp