Re: [PATCH v2 0/3] leds: blink resolution improvements

[Jacek Anaszewski]

On 04/28/2015 03:26 PM, Stas Sergeev wrote:
> 28.04.2015 15:58, Jacek Anaszewski ÐÐÑÐÑ:
> > On 04/28/2015 12:12 PM, Stas Sergeev wrote:
> > > 28.04.2015 11:57, Jacek Anaszewski ÐÐÑÐÑ:
> > > > Hi Stas,
> > > >
> > > > Have you tested it?
> > > Of course I did.
> > > Works with gpio driver and provides up to 10usec precision on
> > > armada-xp board.
> > > This is 1000 times better than without my patch - the precision
> > > was 10ms (jiffy).
> > Please take into account that this could work reliably only for gpio
> > LEDs. For the LEDs driven though a bus (e.g. I2C) delays below 1ms
> > might be hard to achieve. The minimum available delay would vary from
> > driver to driver.
> >
> > We could think of adding the hr_timer mode to the led-class.
> > The mode could be turned on with use of a new led_set_high_res_timer
> > API. The API would be called by drivers/leds/leds-gpio.c driver when
> > a dedicated sysfs attribute was set adequately.
> > The other drivers could also set this mode if they controlled device
> > with a suitable LED switching rate. The minimum delay value could
> > be made configurable by the driver and readable through sysfs
> > when in hr_timer mode.
> Why such a complexity?
> Wouldn't it be enough if the driver can set the minimum delay
> value, yet to always use the hrtimer?

Hr timers would be advantageous only for leds-gpio AFAICS. There is no
reason for making them default for all LED class drivers. I assume that
they provide higher resolution at a cost of consuming more CPU/system
resources.

> Please note that 10ms (jiffy) is an inadequate minimum delay
> pretty much for any driver, I guess. At least 1ms should be possible,
> because, well, you can write 1ms to sysfs attribute even without
> my patch. So even the existing options need hrtimer to work right.

As I mentioned before even with hr timer it wouldn't be possible
to assure that 1ms is achievable due to locking and bus latency
reasons.

> > > > I tried it with Samsung M0 board and
> > > > my leds-aat1290 driver. It didn't work well. And for small delay
> > > > intervals it will not have a chance to work reliably with all drivers,
> > > > especially the ones which use mutex in their brightness_set op,
> > > > since mutex can sleep.
> > > OK, I can remove the nsec resolution.
> > usec also didn't work, please look at my use case and warning:
> >
> > echo "timer" > trigger
> > echo 1 > delay_on
> > echo 1 > delay_off
> > echo usec > delay_unit
> > [  178.584433] hrtimer: interrupt took 300747 ns
> I think I should try a tasklet then.
>
> > Only some time later I realized that for AAT1290 brightness is set
> > through ASCwire pulse protocol, which takes few ms.
> >
> > Please note that with this approach users would have to wonder why
> > they are getting the warnings and why they can't get their LEDs to work
> > with given settings.
> OK, so we need a tasklet and the ability for the driver to set the
> minimal delay. I think I can implement the former and hope someone
> else will later implement the later. :)
> Lets just solve the problem step-by-step. I can't solve all the
> problems at once, but you can't deny the fact that the problem
> exists and needs to be solved.

This seems not to be an urgent issue. Therefore it is better to
provide complex solution in one patch set.


--
Best Regards,
Jacek Anaszewski
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