On Tue, Sep 26, 2023 at 03:43:18PM +0300, Ivaylo Dimitrov wrote: > On 26.09.23 г. 10:16 ч., Sean Young wrote: > > On Mon, Sep 25, 2023 at 07:06:44PM +0300, Ivaylo Dimitrov wrote: > > > On 1.09.23 г. 17:18 ч., Sean Young wrote: > > > > The ir-rx51 is a pwm-based TX driver specific to the N900. This can be > > > > handled entirely by the generic pwm-ir-tx driver, and in fact the > > > > pwm-ir-tx driver has been compatible with ir-rx51 from the start. > > > > > > > > > > Unfortunately, pwm-ir-tx does not work on n900. My investigation shows that > > > for some reason usleep_range() sleeps for at least 300-400 us more than what > > > interval it is requested to sleep. I played with cyclictest from rt-tests > > > package and it gives similar results - increasing the priority helps, but I > > > was not able to make it sleep for less that 300 us in average. I tried > > > cpu_latency_qos_add_request() in pwm-ir-tx, but it made no difference. > > > > > > I get similar results on motorola droid4 (OMAP4), albeit there average sleep > > > is in 200-300 us range, which makes me believe that either OMAPs have issues > > > with hrtimers or the config we use has some issue which leads to scheduler > > > latency. Or, something else... > > > > The pwm-ir-tx driver does suffer from this problem, but I was under the > > impression that the ir-rx51 has the same problem. > > > > Could you elaborate on the "pwm-ir-tx driver does suffer from this problem"? > Where do you see that? So on a raspberry pi (model 3b), if I use the pwm-ir-tx driver, I get random delays of up to 100us. It's a bit random and certainly depends on the load. I'm measuring using a logic analyzer. There have been reports by others on different machines with random delays and/or transmit failures (as in the receiver occassionally fails to decode the IR). I usually suggest they use the gpio-ir-tx driver, which does work as far as I know (the signal looks perfect with a logic analyzer). So far I've taken the view that the driver works ok for most situations, since IR is usually fine with upto 100us missing here or there. The gpio-ir-tx driver works much better because it does the entire send under spinlock - obviously that has its own problems, because an IR transmit can be 10s or even 100s of milliseconds. I've never known of a solution to the pwm-ir-tx driver. If using hrtimers directly improves the situation even a bit, then that would be great. > ir-rx51 does not suffer from the same problem (albeit it has its own one, > see bellow) > > > > In either case help is appreciated to dig further trying to find the reason > > > for such a big delay. > > > > pwm-ir-tx uses usleep_range() and ir-rx51 uses hrtimers. I thought that > > usleep_range() uses hrtimers; however if you're not seeing the same delay > > on ir-rx51 then maybe it's time to switch pwm-ir-tx to hrtimers. > > > > usleep_range() is backed by hrtimers already, however the difference comes > from how hrtimer is used in ir-rx51: it uses timer callback function that > gets called in softirq context, while usleep_range() puts the task in > TASK_UNINTERRUPTIBLE state and then calls schedule_hrtimeout_range(). For > some reason it takes at least 200-400 us (on average) even on OMAP4 to > switch back to TASK_RUNNING state. > > The issue with ir-rx51 and the way it uses hrtimers is that it calls > pwm_apply_state() from hrtimer function, which is not ok, per the comment > here > https://elixir.bootlin.com/linux/v6.6-rc3/source/drivers/pwm/core.c#L502 > > I can make pwm-ir-tx switch to hrtimers, that's not an issue, but I am > afraid that there is some general scheduler or timers (or something else) > issue that manifests itself with usleep_range() misbehaving. If we can switch pwm-ir-tx to hrtimers, that would be great. The ir-rx51 removal patches have already been queued to media_staging; we may have to remove them from there if we can't solve this problem. > > I don't have a n900 to test on, unfortunately. > > > > I have and once I have an idea what's going on will port pwm-ir-tx to > hrtimers, if needed. Don't want to do it now as I am afraid the completion I > will have to use will have the same latency problems as usleep_range() That would be fantastic. Please do keep us up to date with how you are getting on. Like I said, it would be nice to this resolved before the next merge window. Thanks, Sean
