linux-kernel - Re: [PATCH/RFC v8 02/14] Documentation: leds: Add description of LED Flash class extension

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Date:	Wed, 10 Dec 2014 14:17:15 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Bryan Wu <cooloney@...il.com>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	b.zolnierkie@...sung.com, "rpurdie@...ys.net" <rpurdie@...ys.net>,
	Sakari Ailus <sakari.ailus@....fi>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>
Subject: Re: [PATCH/RFC v8 02/14] Documentation: leds: Add description of LED
 Flash class extension

On 12/09/2014 04:50 PM, Pavel Machek wrote:
> On Tue 2014-12-09 09:54:06, Jacek Anaszewski wrote:
>> Hi Pavel,
>>
>> On 12/08/2014 09:18 PM, Pavel Machek wrote:
>>> On Mon 2014-12-08 17:55:20, Jacek Anaszewski wrote:
>>>> On 12/06/2014 01:43 PM, Pavel Machek wrote:
>>>>>
>>>>>>> The format of a sysfs attribute should be concise.
>>>>>>> The error codes are generic and map directly to the V4L2 Flash
>>>>>>> error codes.
>>>>>>>
>>>>>>
>>>>>> Actually I'd like to see those flash fault code defined in LED
>>>>>> subsystem. And V4L2 will just include LED flash header file to use it.
>>>>>> Because flash fault code is not for V4L2 specific but it's a feature
>>>>>> of LED flash devices.
>>>>>>
>>>>>> For clearing error code of flash devices, I think it depends on the
>>>>>> hardware. If most of our LED flash is using reading to clear error
>>>>>> code, we probably can make it simple as this now. But what if some
>>>>>> other LED flash devices are using writing to clear error code? we
>>>>>> should provide a API to that?
>>>>>
>>>>> Actually, we should provide API that makes sense, and that is easy to
>>>>> use by userspace.
>>>>>
>>>>> I believe "read" is called read because it does not change anything,
>>>>> and it should stay that way in /sysfs. You may want to talk to sysfs
>>>>> maintainers if you plan on doing another semantics.
>>>>
>>>> How would you proceed in case of devices which clear their fault
>>>> register upon I2C readout (e.g. AS3645)? In this case read does have
>>>> a side effect. For such devices attribute semantics would have to be
>>>> different than for the devices which don't clear faults on readout.
>>>
>>> No, semantics should be same for all devices.
>>>
>>> If device clears fault register during I2C readout, kernel will simply
>>> gather faults in an variable, and clear them upon write to sysfs file.
>>
>> This approach would require implementing additional mechanisms on
>> both sides: LED Flash class core and a LED Flash class driver.
>> In the former the sysfs attribute write permissions would have
>> to be decided in the runtime and in the latter caching mechanism
>
> Write attributes at runtime? Why? We can emulate sane and consistent
> behaviour for all the controllers: read gives you list of faults,
> write clears it. We can do it for all the controllers.

I don't like the idea of listing the faults in the form of strings.
I'd like to see the third opinion :)

> Only cost is few lines of code in the drivers where hardware clears
> faults at read.

As above - the third opinion would be appreciated.

>> would have to be implemented per driver. We would have to also
>> consider how to approach the issue in case of sub-leds.
>
> Actually.. sub-leds. That is one physical LED being connected to two
> current sources at the same time, right?

There are possible designs with two separate LEDs.

-- 
Best Regards,
Jacek Anaszewski
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