linux-kernel - Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

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Message-ID: <a7975f8d-e2e2-6554-4bab-af8c00dc9252@baylibre.com>
Date:   Mon, 12 Sep 2016 17:03:32 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     David Rivshin <drivshin@...rd.com>
Cc:     Matthijs van Duin <matthijsvanduin@...il.com>,
        "H. Nikolaus Schaller" <hns@...delico.com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>, marek@...delico.com,
        linux-omap@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, letux-kernel@...nphoenux.org
Subject: Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM
 frequency to 83Hz

On 09/12/2016 04:41 PM, David Rivshin wrote:
> On Sat, 10 Sep 2016 15:48:28 +0200
> Neil Armstrong <narmstrong@...libre.com> wrote:
> 
>> Le 10/09/2016 05:17, Matthijs van Duin a écrit :
>>> On Mon, Sep 05, 2016 at 11:16:38AM +0200, H. Nikolaus Schaller wrote:  
>>>> This helps to get 100% intensity closer to "always on".
[...]
>>> }
>>>
>>> At the time I used a scope to check the exact behaviour of dmtimer pwm
>>> on a dm814x.  My notes mention (when pwm enabled):
>>> 	match < reload	output on continuous
>>> 	match == reload	output on 1 cycle, off period-1 cycles
>>> 	match == -2	output on period-1 cycles, off 1 cycle
>>> 	match == -1	output freezes
>>>
>>> Hope this helps  
>>
>> Hi,
>>
>> I think these corner cases should definitely be handled in the dmtimer driver.
> 
> Do you mean to modify the dmtimer driver itself, or the pwm-omap-dmtimer 
> driver?
> 
> IIRC from the last time I was in the pwm-omap-dmtimer driver, it seemed to 
> me that the 0% and 100% cases could/should be handled as simple special 
> cases there. I think the dmtimer driver itself has the necessary API to the 
> HW, but I'd need to re-familiarize myself with it to remember the details 
> of what I was thinking. 
> 
> Actually, I did mention some thoughts on this a previous thread where
> Adam Ford was using pwm-omap-dmtimer for a backlight:
>  http://www.spinics.net/lists/linux-omap/msg126006.html
> So it may be as simple as using PWM_OMAP_DMTIMER_TRIGGER_NONE and passing 
> def_on according to whether 0 or 100% duty were requested (and polarity).

Yes it's exactly what I was talking about.

> 
> 
>>
>> I'll try to post a fix to handle these, thanks for the original code dump.
>>
>>>
>>> Matthijs
>>>   
>>
>> Neil

Thanks,
Neil