linux-kernel - Re: [PATCH v5 1/2] leds: core: Introduce LED pattern trigger

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Date:   Fri, 24 Aug 2018 22:44:47 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Baolin Wang <baolin.wang@...aro.org>, rteysseyre@...il.com,
        bjorn.andersson@...aro.org, broonie@...nel.org,
        linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] leds: core: Introduce LED pattern trigger

On 08/24/2018 10:12 PM, Pavel Machek wrote:
> On Fri 2018-08-24 21:49:50, Jacek Anaszewski wrote:
>> Hi Pavel,
>>
>> On 08/24/2018 12:11 PM, Pavel Machek wrote:
>>> Hi!
>>>
>>>> I think that it would be more flexible if software pattern fallback
>>>> was applied in case of pattern_set failure. Otherwise, it would
>>>> lead to the situation where LED class devices that support hardware
>>>> blinking couldn't be applied the same set of patterns as LED class
>>>> devices that don't implement pattern_set. The latter will always have to
>>>> resort to using software pattern engine which will accept far greater
>>>> amount of pattern combinations.
>>>>
>>>> In this case we need to discuss on what basis the decision will be
>>>> made on whether hardware or software engine will be used.
>>>>
>>>> Possible options coming to mind:
>>>> - an interface will be provided to determine max difference between
>>>>   the settings supported by the hardware and the settings requested by
>>>>   the user, that will result in aligning user's setting to the hardware
>>>>   capabilities
>>>> - the above alignment rate will be predefined instead
>>>> - hardware engine will be used only if user requests supported settings
>>>>   on the whole span of the requested pattern
>>>> - in each of the above cases it would be worth to think of the
>>>>   interface to show the scope of the settings supported by hardware
>>>
>>> I'd recommend keeping it simple. We use hardware engine if driver
>>> author thinks pattern is "close enough".
>>
>> The thing is that in the ledtrig-pattern v5 implementation there
>> is no option of using software fallback if pattern_set op
>> is initialized:
>>
>> +	if (led_cdev->pattern_set) {
>> +		return led_cdev->pattern_set(led_cdev, data->patterns,
>> +					     data->npatterns, data->repeat);
>> +	}
> 
> Yeah, that sounds wrong. (Sorry I did not pay enough attention).
> 
> It pattern_set() returns special error code, it should just continue
> and use software pattern fallback.

And now we can get back to the issue I was concerned about in the
email you replied to, i.e. what series of [brightness delta_t] tuples
should be written to the pattern file to enable hardware breathing
engine.

-- 
Best regards,
Jacek Anaszewski