On Tue, Apr 13, 2021 at 09:45:29AM -0300, Jonas Malaco wrote: > Guenter (and others on this list), Very gentle ping. I also thought posting these questions first would be less disruptive than a RFC patch, but please let me know if you prefer the latter. Thanks again, Jonas > > I am getting ready to submit a driver for NZXT Grid+ V3 and Smart Device > (V1) fan controllers, but I am having trouble deciding how to expose > their PWM control due to some device limitations. > > Before getting into those, let me first give some very basic context... > > These devices are USB HIDs, and asynchronously send "status" reports > every 200 ms to communicate speed, current, voltage and control mode for > their channels (one channel per report). > > Fans can be controlled by sending HID output reports to the device, and > both DC and PWM modes are supported. The device features a special > initialization routine (that must be requested during probe) which > automatically detects the appropriate control mode for each channel. > > Back to the device limitations... > > The first is that PWM values can be set, but not read back. And neither > hwmon[1] nor lm-sensors' pwmconfig/fancontrol expect pmw* attributes to > be WO. One solution is to have the driver track the PWM values that are > set through it and return those, but is this acceptable? > > The other starts with PWM control being disabled for channels that the > device identifies as unconnected. This is not in itself a problem, but > the initialization routine (where the detection happens) is > asynchronous, takes somewhere around 5 seconds, and we do not have any > way of directly querying its result. We only know the control mode of > each channel (be it DC, PWM or disabled) from the regular status > reports. > > These limitations make it complicated to simply use is_visible() to hide > pwm attributes of unconnected channels. We would need to register with > the hwmon subsystem only after getting enough post-initialization status > reports for all channels, and this would essentially mean to sleep for > 6+ seconds. We would also need to unregister and re-register when going > through a suspend-reset_resume cycle, because the device may have its > state wiped, requiring reinitialization.[2] > > A different approach to handle this, which I have preferred _so far,_ is > to use pwm*_enable = 0 to report the unconnected channels to user-space, > while keeping the other pwm attributes visible. But this comes with > other problems. > > First, lm-sensors' pwmconfig expects to be able to write to a > pwm*_enable attribute if it exists, but the device does not support that > operation. The hwmon documentation states that RW values may be RO, but > pwmconfig is out there and in use. So far I simply return 0 to attempts > at those writes, silently ignoring them; functional, but certainly a > hack. > > Second, if PWM control is disabled for a channel, but its pwm* and > pwm*_mode attributes are still visible, what should we return when > user-space attempts to write to them? The practical answer may simply > be to return -EOPNOTSUPP, but this makes me wonder if the whole approach > (of handling these cases with pwm*_enable instead of is_visible()) is > not doomed. > > A final minor problem is that channels detected as unconnected run at > 40% PWM, but the documentation for pwm*_enable == 0 is a bit too > specific: "no fan speed control (i.e. fan at *full* speed)" (emphasis > mine). > > Do you have any suggestions and/or recommendations? > > If it helps, a pre-RFC (but functional and mostly clean) version of the > driver can be found at: > > https://github.com/jonasmalacofilho/linux/blob/p-hwmon-add-nzxt-smartdevice-gridplus3/drivers/hwmon/nzxt-smartdevice.c > > Thanks, > Jonas > > [1] According to Documentation/hwmon/sysfs-interface.rst. > [2] The device also does not respond to HID Get_Report, so it is not > trivial to check whether it really needs to be reinitialized, since > the only symptom of that being necessary is the absence of the > asynchronous status reports.
