From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jacek Anaszewski Subject: Re: [PATCH v2 0/3] leds: blink resolution improvements Date: Wed, 29 Apr 2015 17:06:32 +0200 Message-ID: <5540F378.9010106@samsung.com> References: <553E6CF5.4030601@list.ru> <553F4B60.20204@samsung.com> <553F5CFF.9090601@list.ru> <553F83DC.8080701@samsung.com> <553F8A7F.7040700@list.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Received: from mailout1.w1.samsung.com ([210.118.77.11]:37822 "EHLO mailout1.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423038AbbD2PGg (ORCPT ); Wed, 29 Apr 2015 11:06:36 -0400 In-reply-to: <553F8A7F.7040700@list.ru> Sender: linux-leds-owner@vger.kernel.org List-Id: linux-leds@vger.kernel.org To: Stas Sergeev Cc: linux-leds@vger.kernel.org, Linux kernel , Stas Sergeev On 04/28/2015 03:26 PM, Stas Sergeev wrote: > 28.04.2015 15:58, Jacek Anaszewski =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >> On 04/28/2015 12:12 PM, Stas Sergeev wrote: >>> 28.04.2015 11:57, Jacek Anaszewski =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>> 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 fro= m >> 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 driv= ers, >>>> 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 w= ork >> 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. --=20 Best Regards, Jacek Anaszewski