On Wed, Apr 21, 2021 at 01:48:03PM -0300, Jonas Malaco wrote: > 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. > It is a difficult subject, and I am struggling myself with the situations you are presenting. > 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? I have seen a couple of those recently. I think returning -ENODATA if the value isn't known (yet) is the best possible solution. I thought about adding that to the ABI, actually. > > > > 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. Again, I think the best solution is to return -ENODATA until the value is known. > > > > 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] > > I think the above should resolve that. > > 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. It is a bad idea to return 0 if the value is not accepted. You could check if the written value matches the current value and return 0 if it does, and an error such as -EOPNOTSUPP or -EINVAL otherwise. > > > > 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. > > Mode isn't really writeable either, isn't it ? If so, use the same trick as with the _enable attribute. The same is effectively true for the pwm value itself: Since both _enable and _mode are effectively read-only, you can accept a write only if fan control is enabled, and return an error if it isn't. > > 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). Just document the difference. Reality doesn't always match our expectations. Thanks, Guenter > > > > 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.
