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Date:   Mon, 10 Jun 2019 13:39:28 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc:     Pavel Machek <pavel@ucw.cz>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Doug Anderson <dianders@google.com>,
        Rob Herring <robh+dt@kernel.org>,
        Jingoo Han <jingoohan1@gmail.com>,
        Richard Purdie <rpurdie@rpsys.net>,
        Jacek Anaszewski <jacek.anaszewski@gmail.com>,
        Brian Norris <briannorris@google.com>,
        Guenter Roeck <groeck@google.com>,
        Lee Jones <lee.jones@linaro.org>,
        Alexandru Stan <amstan@google.com>, linux-leds@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com
Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED
 linearly to human eye.
Message-ID: <20190610203928.GA137143@google.com>
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Hi Enric

On Mon, Jun 10, 2019 at 12:00:02PM +0200, Enric Balletbo i Serra wrote:
> Hi Matthias,
> 
> On 8/6/19 23:02, Pavel Machek wrote:
> > Hi!
> > 
> >>> +	 * Note that this method is based on empirical testing on different
> >>> +	 * devices with PWM of 8 and 16 bits of resolution.
> >>> +	 */
> >>> +	n = period;
> >>> +	while (n) {
> >>> +		counter += n % 2;
> >>> +		n >>= 1;
> >>> +	}
> >>
> >> I don't quite follow the heuristics above. Are you sure the number of
> >> PWM bits can be infered from the period? What if the period value (in
> >> ns) doesn't directly correspond to a register value? And even if it
> >> did, counting the number of set bits (the above loops is a
> >> re-implementation of ffs()) doesn't really result in the dividers
> >> mentioned in the comment. E.g. a period of 32768 ns (0x8000) results
> >> in a divider of 1, i.e. 32768 brighness levels.
> >>
> 
> Right, I think that only works on the cases that we only have one pwm cell, and
> looks like during my tests I did only tests on devices with one pwm cell :-(
> 
> And as you point the code is broken for other cases (pwm-cells > 1)
> 
> >> On veyron minnie the period is 1000000 ns, which results in 142858
> >> levels (1000000 / 7)!
> >>
> >> Not sure if there is a clean solution using heuristics, a DT property
> >> specifying the number of levels could be an alternative. This could
> >> also be useful to limit the number of (mostly) redundant levels, even
> >> the intended max of 4096 seems pretty high.
> >>
> 
> Looking again looks like we _can not_ deduce the number of bits of a pwm, it is
> not exposed at all, so I think we will need to end adding a property to specify
> this. Something similar to what leds-pwm binding does, it has:
> 
> max-brightness : Maximum brightness possible for the LED

Thanks for the confirmation that I didn't just miss some clever trick.

I also think that some kind of DT property is needed, I'll try to come
up with a reasonable name, keeping in mind that some devices might not
want to use the entire range of levels.