Re: [PATCH RFC] arm64: dts: allwinner: a64: teres-i: Enable audio

[Harald Geyer <harald@ccbib.org>]

Hi Maxime!

Maxime Ripard writes:
> On Tue, Feb 12, 2019 at 08:37:36PM +0100, Harald Geyer wrote:
> > > There's a few issues with that approach as well:
> > > 
> > >   - We're actively trying to remove the pinctrl nodes for the GPIOs
> > 
> > For what reason? Maybe it doesn't apply to this usecase?
> 
> This is kind of separate. At the moment, on all our SoCs but the H6,
> requesting a pin to a separate state using pinctrl doesn't mark the
> GPIO muxed on that pin as reserved, so through the GPIO userspace
> interface (or calling gpio_request from within the kernel, but that
> seems less of a risk) anyone is free to just request a GPIO on a pin
> already requested, behind the consumer drivers' back. Which is pretty
> bad.

Really, I'm surprised. This is not the behaviour I remember from A20
and A64. Indeed, testing this on teres with the debug detect pin claimed
by audio, I get:

root@teres:/sys/kernel/debug/pinctrl/1f02c00.pinctrl# cat pinmux-pins
Pinmux settings per pin
Format: pin (name): mux_owner|gpio_owner (strict) hog?
pin 352 (PL0): device 1f03400.rsb function s_rsb group PL0
pin 353 (PL1): device 1f03400.rsb function s_rsb group PL1
pin 354 (PL2): GPIO 1f02c00.pinctrl:354
pin 355 (PL3): UNCLAIMED
pin 356 (PL4): UNCLAIMED
pin 357 (PL5): UNCLAIMED
pin 358 (PL6): UNCLAIMED
pin 359 (PL7): GPIO 1f02c00.pinctrl:359
pin 360 (PL8): GPIO 1f02c00.pinctrl:360
pin 361 (PL9): device sound function gpio_out group PL9
pin 362 (PL10): UNCLAIMED
pin 363 (PL11): UNCLAIMED
pin 364 (PL12): GPIO 1f02c00.pinctrl:364
[...]

root@teres:/sys/class/gpio# echo 361 >export
bash: echo: Schreibfehler: Das Argument ist ungültig.

So I can't access this from sysfs, even though the error code is a
bit odd: I'd expect EBUSY instead of EINVAL. I can export any of the
UNCLAIMED pins/gpios.

Trying with libgpiod as well, I see that the state of the pin is reported
incorretly, but I still can't access it:

gpiochip0 - 32 lines:
        line   0:      unnamed       unused   input  active-high
        line   1:      unnamed       unused   input  active-high
        line   2:      unnamed      "reset"  output   active-low [used]
        line   3:      unnamed       unused   input  active-high
        line   4:      unnamed       unused   input  active-high
        line   5:      unnamed       unused   input  active-high
        line   6:      unnamed       unused   input  active-high
        line   7:      unnamed  "usb1-vbus"  output  active-high [used]
        line   8:      unnamed "Lid Switch"   input   active-low [used]
        line   9:      unnamed       unused   input  active-high
        line  10:      unnamed      "sysfs"   input  active-high [used]
        line  11:      unnamed       unused   input  active-high
        line  12:      unnamed     "enable"  output  active-high [used]
        line  13:      unnamed       unused   input  active-high

root@teres:~# gpioget 1f02c00.pinctrl 9
gpioget: error reading GPIO values: Invalid argument

On a pin exported to sysfs I get EBUSY as expected:
root@teres:~# gpioget 1f02c00.pinctrl 10
gpioget: error reading GPIO values: Device or resource busy

And reading an unclaimed pin works as expected too:
root@teres:~# gpioget 1f02c00.pinctrl 11
0

Either I misunderstood what you have written or it isn't true.

> There's support for such a check in pinctrl, and we did enable it for
> the H6. However, one of its side effect is that you can't have a
> pinctrl node for a GPIO anymore (at least without significantly
> reworking the GPIO API in the kernel).

Can you point me to some background reading?

> We did enable it for the H6, since it didn't have any backward
> compatibility to take care of, but it's disabled at the moment for all
> the other SoCs to be able to flip that switch at some point. And
> that's why we're moving away from it as well.

Well ... that's good to know, because I have a couple of custom DTs
with pinctrl nodes for a GPIO. I think it should be documented as
deprecated in the binding then.

Also I wonder how I can select drive strength or bias on a gpio line when
I can't use pinctrl with them anymore.

> > I think the real downside of this approach is, that using the UART
> > makes the internal speakers/mic unuseable too.
> 
> That's also a pretty big issue.

I certainly agree it's unfortunate.

> > But we need a way to control the mux from userspace and aside from
> > unbinding the ideas proposed thus far are:
> >
> > a) control the gpio directly
> > b) control the gpio via leds-gpio
> > 
> > (a) was dismissed because we can't set a default from DT
> > (b) was dismissed because some rogue app might try to blink it
> > 
> > The clean solution might be to write mux-gpio, which is actually
> > identical to leds-gpio but lives in /sys/class/mux_switches/ and
> > uses different filenames. But that's going down the "invent a new
> > subsystem road", which I believe is overkill for what is a debugging
> > facility for a single board.
> 
> I still believe we should aim at supporting this through pinctrl, and
> adding an userspace API is definitely easier than a full subsystem.

Getting everybody to agree on a new API (especially a userspace ABI)
is a major headache (and rightly so, we want to get something right on
the first attempt that is going to stay around forever). I don't think
some quirky debugging feature is worth the effort.

And frankly I don't care much about audio on the teres. I started
working on this because I feel kind of responsible for keeping the
teres DT up-to-date with what the kernel can support. But if the
kernel can't support it ATM: so be it.

As a compromise I think we could add all the nodes to the DT but mark
their status as "disabled". That would help everybody wanting to enable
audio but still be technically correct.

Harald

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