linux-kernel - Re: Reading /sys with side effects (was Re: [PATCH 1/2] Documentation: leds: Add description of LED Flash class extension)

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Date:	Mon, 2 Feb 2015 14:51:03 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
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!

> >[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.
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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