On Thu, Nov 03, 2022 at 05:40:44AM +0000, Billy Tsai wrote: > On 2022/11/3, 12:30 PM, "Guenter Roeck" <groeck7@xxxxxxxxx on behalf of linux@xxxxxxxxxxxx> wrote: > > On Thu, Nov 03, 2022 at 03:52:59AM +0000, Billy Tsai wrote: > > > > > > Can't I use a min/max RPM to let the driver know a reasonable timeout/polling period when > > > the driver is trying to get RPM? > > > Beacause that our tach controller have the falg to indicates the hardware detected the change > > > in the input signal. I need the proper timout to rule out slow RPMs. > > > If the chip measures the fan speed continuously, why would that ever be a > > problem, and why wait in the first place instead of just taking the most > > recent result ? > > > Pretty much every other driver is doing that, so I really don't understand > > why that would not work here. > > When the fan speed drop from a very fast RPM to a very slow RPM. Especially when it is close to stopping. > The most recent result will be no meaningful value. The slower RPM needs more time to sample it. E.g., If > we want to measure the fan with 600 RPM, the controller needs at least 100ms. During this time period, we > will always get the wrong value. So, our tach controller have the flag to avoid this problem: > TACH_ASPEED_VALUE_UPDATE: tach value updated since last read > This flag will be set when the controller detected the change of the signal and clear by read it. > In order to use this flag, the controller needs the proper timeout based on minimum RPM to avoid waiting forever. > I am not going to accept this patch as-is. If userspace wants to have values accurate down to ms, this kind of approach is just wrong. Users will have to live with the fact that measurements may be a bit (in the < 1 second range) out of date. Many older drivers even implement code which avoids reading registers again for a second or longer. Older temperature sensors may take several seconds to provide new readings. That is not a reason to block userspace until a new value is available. I do not see that as a problem. In my opinion it is much more of a problem if the driver returns a completely bad value such as 0 or even an error code because its software parameters did not match reality and the driver didn't wait long enough for a new value. That would be _much_ worse than providing a value which is a few 100 ms out of date, and your code is vulnerable to that problem. Besides, for a fan to reduce its speed that quickly, it has to be manually stopped. Normally fans take several seconds to stop if power is taken away completely. Your code is adding a lot of complexity (and unnecessary attributes) for no good reason. Guenter
