Messages in this thread | | | Subject | Re: [PATCH v5 2/2] media: rc: remove ir-rx51 in favour of generic pwm-ir-tx | From | Ivaylo Dimitrov <> | Date | Fri, 29 Sep 2023 11:49:52 +0300 |
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Hi,
On 26.09.23 г. 23:18 ч., Sean Young wrote: > 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. >
The issue with hrtimers is that we cannot use them directly, as pwm_apply_state() may sleep, but hrtimer function is called in atomic context.
>> 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. >
I made some POC here, but unfortunately it failed more or less. The idea of POC is: setup hrtimer, start it in pwm_ir_tx() and do wait_for_completion() in a loop while calling complete() for the timer function. While it improves things a bit, I wouldn't say it makes the driver working ok on n900 - my TV registers one of let's say 5-10 pulse packages.
We have couple of issues:
- scheduler seems to use 32kHz timer, which means that we can never have precise pulse width, with error up to ~30 us, no matter what we do, IIUC.
- wait_for_completion() suffers from the same latency issue that usleep_range() has - it exits after 300-400 us after complete() has been called in the timer function.
- turning pwm off needs ~300us, because of either omap_dm_timer_stop() calling clk_get_rate() or __omap_dm_timer_stop() waiting for fclk period * 3.5 (see https://elixir.bootlin.com/linux/v6.6-rc3/source/drivers/clocksource/timer-ti-dm.c#L269)
- in order to achieve some sane latency distribution, I have to set_user_nice(current, MIN_NICE); in pwm_ir_tx()
> 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. >
ir-rx51 has conceptual problem of calling function that might sleep from atomic context, however, we can fix omap_dm_timer_stop() to not call clk_get_rate() and that would make it working. So yeah, if we can't fix pwm-ir-tx then patches removal along with fixing dmtimer and fixing a couple of code issues ir-rx51 has, seems the only option to have working IRTX on n900. Maybe we can rename it to pwm-ir-tx-hrtimer as there is nothing n900 specific in it.
>>> 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. >
The only thing I didn't try yet is to start another thread and to set that thread to use FIFO scheduler. I will report once I have tried that.
Regards, Ivo
> Thanks, > Sean >
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