Re: [PATCH 1/2 v2] serial: mctrl_gpio: Check if GPIO property exisits before requesting it

From: Stefan Roese
Date: Fri May 24 2019 - 07:32:53 EST

On 24.05.19 13:11, Andy Shevchenko wrote:
On Fri, May 24, 2019 at 1:21 PM Mika Westerberg
<mika.westerberg@xxxxxxxxxxxxxxx> wrote:

On Fri, May 24, 2019 at 11:48:24AM +0200, Stefan Roese wrote:
This patch adds a check for the GPIOs property existence, before the
GPIO is requested. This fixes an issue seen when the 8250 mctrl_gpio
support is added (2nd patch in this patch series) on x86 platforms using

Here Mika's comments from 2016-08-09:

I noticed that with v4.8-rc1 serial console of some of our Broxton
systems does not work properly anymore. I'm able to see output but input
does not work.

I bisected it down to commit 4ef03d328769eddbfeca1f1c958fdb181a69c341
("tty/serial/8250: use mctrl_gpio helpers").

The reason why it fails is that in ACPI we do not have names for GPIOs
(except when _DSD is used) so we use the "idx" to index into _CRS GPIO
resources. Now mctrl_gpio_init_noauto() goes through a list of GPIOs
calling devm_gpiod_get_index_optional() passing "idx" of 0 for each. The
UART device in Broxton has following (simplified) ACPI description:

Device (URT4)
Name (_CRS, ResourceTemplate () {
GpioIo (Exclusive, PullDefault, 0x0000, 0x0000, IoRestrictionOutputOnly,
"\\_SB.GPO0", 0x00, ResourceConsumer)
GpioIo (Exclusive, PullDefault, 0x0000, 0x0000, IoRestrictionOutputOnly,
"\\_SB.GPO0", 0x00, ResourceConsumer)

In this case it finds the first GPIO (0x003A which happens to be RX pin
for that UART), turns it into GPIO which then breaks input for the UART
device. This also breaks systems with bluetooth connected to UART (those
typically have some GPIOs in their _CRS).

Any ideas how to fix this?

We cannot just drop the _CRS index lookup fallback because that would
break many existing machines out there so maybe we can limit this to
only DT enabled machines. Or alternatively probe if the property first
exists before trying to acquire the GPIOs (using

This patch implements the fix suggested by Mika in his statement above.

We have a board where ASL provides _DSD for CTS and RxD pins.
I'm afraid this won't work on it.

With "won't work" you mean, that the GPIO can't be used for modem
control in this case in the current implementation (with this
patchset)? Or do you mean, that the breakage (input does not work
on Broxton systems) will not be solved by this patch?

If its the former, then I think that solving this issue is something
for a new patch, to support modem-control on such platforms as well
(if needed).

Please note that this patch is not trying to get modem-control working
on such ACPI based systems. Its targeted for device-tree enabled
platforms, using the 8250 serial driver, here specifically a MIPS
MT7688 based board. And just wants to fix the latter issue mentioned
above so that the 8250 modem-control support can be accepted in
Basically we need to understand the use of the GPIOs in UART. In our
case it's an out-of-band wake up source for UART.
Simply requiring GPIOs to be present is not enough.

Perhaps property like 'modem-control-gpio-in-use' (this seems a bad
name, given for sake of example).