Re: [PATCH] pinctrl: qcom: Fix behavior in abscense of open-drain support

[Johan Hovold <johan@kernel.org>]

On Wed, Apr 24, 2024 at 08:45:31PM -0700, Bjorn Andersson wrote:
> When a GPIO is configured as OPEN_DRAIN gpiolib will in
> gpiod_direction_output() attempt to configure the open-drain property of
> the hardware and if this fails fall back to software emulation of this
> state.
> 
> The TLMM block in most Qualcomm platform does not implement such
> functionality, so this call would be expected to fail. But due to lack
> of checks for this condition, the zero-initialized od_bit will cause
> this request to silently corrupt the lowest bit in the config register
> (which typically is part of the bias configuration) and happily continue
> on.
> 
> Fix this by checking if the od_bit value is unspecified and if so fail
> the request to avoid the unexpected state, and to make sure the software
> fallback actually kicks in.

Fortunately, this is currently not a problem as the gpiochip driver does
not implement the set_config() callback, which means that the attempt to
change the pin configuration currently always fails with -ENOTSUP (see
gpio_do_set_config()).

Specifically, this means that the software fallback kicks in, which I
had already verified.

Now, perhaps there is some other path which can allow you to end up
here, but it's at least not via gpiod_direction_output().

The msm pinctrl binding does not allow 'drive-open-drain' so that path
should also be ok unless you have a non-conformant devicetree.

> It is assumed for now that no implementation will come into existence
> with BIT(0) being the open-drain bit, simply for convenience sake.
> 
> Fixes: 13355ca35cd1 ("pinctrl: qcom: ipq4019: add open drain support")

I guess hardware open-drain mode has never been properly tested on
ipq4019.

> Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
> ---
>  drivers/pinctrl/qcom/pinctrl-msm.c | 2 ++
>  drivers/pinctrl/qcom/pinctrl-msm.h | 3 ++-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pinctrl/qcom/pinctrl-msm.c b/drivers/pinctrl/qcom/pinctrl-msm.c
> index aeaf0d1958f5..329474dc21c0 100644
> --- a/drivers/pinctrl/qcom/pinctrl-msm.c
> +++ b/drivers/pinctrl/qcom/pinctrl-msm.c
> @@ -313,6 +313,8 @@ static int msm_config_reg(struct msm_pinctrl *pctrl,
>  			*mask |= BIT(g->i2c_pull_bit) >> *bit;
>  		break;
>  	case PIN_CONFIG_DRIVE_OPEN_DRAIN:
> +		if (!g->od_bit)
> +			return -EOPNOTSUPP;

I believe this should be -ENOTSUPP, which the rest of the driver and
subsystem appear to use.

>  		*bit = g->od_bit;
>  		*mask = 1;
>  		break;

Johan