On Fri, Apr 26, 2024 at 03:08:06PM -0700, Brian Norris wrote: > Hi Johan, Bjorn, > > On Thu, Apr 25, 2024 at 5:02 AM Johan Hovold <johan@xxxxxxxxxx> wrote: > > > > 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. > > Apologies if I broke something here. False alarm on the breakage part, I got lost in the software layers. > Both the pinctrl subsystem and > the wide world of diverse QCOM chips can be complicated beasts. I > definitely could have missed things along the way. (And on first > glance, it seems like you may have found one. I definitely did not > consider the gpiod_direction_output() "emulation" behavior here when > submitting this.) > > But I can't tell based on subsequent conversation: are you observing a > real problem, or is this a theoretical one that only exists if the > gpiochip driver adds set_config() support? > There is a problem that if a non-ipq4019 device where to be pinconf'ed for open-drain, the outcome would be unexpected and I have a concern that someone one day would implement set_config(). So, I'd like to fix this, but my argumentation is at least wrong. > > > 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. > > The ipq4019 binding does: > https://git.kernel.org/linus/99d19f5a48ee6fbc647935de458505e9308078e3 > Perhaps we could convert that to yaml? > This is used in OpenWrt device trees. > Thanks, I couldn't find a user, so this was helpful input for deciding the path forward. > > > 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. > > It was quite some time ago that I wrote and tested this, and per the > above, I easily could have missed things. (Plus, the open drain > configuration may not have much practical effect on the systems in > question, so certain errors may not even be observable.) > > But I do recall seeing the code in question activate. And inspection > shows that the pinconf_apply_setting() -> ... msm_config_group_set() > path is non-dead code here, for appropriate device trees. > Thank you for taking a look, Brian. This was valuable input. I will rework this to have a valid motivation - at least. > I can try to fire up my development devices again and see what's up if > that helps, but I won't have time to do that in the next few days. > As my observation was incorrect, I don't think that is urgent. Regards, Bjorn
