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Date: Mon, 02 Feb 2015 15:51:53 +0100 From: Jacek Anaszewski <j.anaszewski@...sung.com> To: Pavel Machek <pavel@....cz> Cc: Greg KH <greg@...ah.com>, kernel list <linux-kernel@...r.kernel.org>, linux-leds@...r.kernel.org, devicetree@...r.kernel.org, kyungmin.park@...sung.com, b.zolnierkie@...sung.com, cooloney@...il.com, rpurdie@...ys.net, sakari.ailus@....fi, s.nawrocki@...sung.com Subject: Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension) Hi Pavel, On 02/02/2015 02:51 PM, Pavel Machek wrote: > Hi! > >>> [Actually, you could _always_ do two reads on those devices, discard >>> first result, and return the second. But I'm not sure how hardware >>> will like that.] >> >> This would be the most sensible option. >> >> >> However, let's analyze the typical use cases for flash strobing: >> >> >> ------------------------------------------------------------- >> >> >>>>>>>>>> Version without faults caching: >> >> ============ >> Driver side: >> ============ >> >> read_faults() >> faults = read_i2c(); //read faults >> if faults >> write_i2c(); //clear faults, only for some devices >> faults = read_i2c(); //read faults >> return faults >> >> ================ >> User space side: >> ================ >> >> 1. faults = `cat flash_faults` //read_faults() >> 2. if faults then >> print "Unable to strobe the flash LED due to faults" >> else >> echo 1 > flash_strobe >> >> >>>>>>>>>> Version with faults caching: >> >> ============ >> Driver side: >> ============ >> >> read_faults() >> faults |= read_i2c(); //read faults >> >> clear_faults() >> write_i2c(); //clear faults >> faults = 0; >> >> >> ================ >> User space side: >> ================ >> >> 1. faults = `cat flash_faults` //read_faults() >> 2. if faults then >> echo 0 > flash_faults //clear_faults() >> faults = `cat flash_faults` //read_faults() >> 3, if !faults >> echo 1 > flash_strobe >> else >> print "Unable to strobe the flash LED due to faults" >> >> >> ------------------------------------------------------------- >> >> From the above it seems that version with clearing faults on read >> results in the simpler flash strobing procedure on userspace side, >> by the cost of additional bus access on the driver side. > > I like caching version more (as it will allow by-hand debugging of > "why did not flash fire? Aha, lets see in the file, there was fault), > but both should be acceptable. > >> we don't need additional attribute, just writing the flash_faults >> attribute can do the clearing. > > Yes, writing flash_faults to clear is acceptable. I've been just inspired with another approach: Faults register is read in the strobe_set callback, right after sending flash strobe command to the device. The userspace can read the cached faults through flash_faults attribute. This way, we avoid reading sysfs attribute with side effect and gain the possibility of giving immediate feedback to the user. Exemplary use case: 1. echo 1 > flash_strobe write_i2c(); //strobe flash faults = read_i2c(); //read faults if faults return -EINVAL; return 0; 2. cat flash_faults return faults; -- Best Regards, Jacek Anaszewski -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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