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.
Date: Mon, 10 Jun 2019 13:39:28 -0700 [thread overview]
Message-ID: <20190610203928.GA137143@google.com> (raw)
In-Reply-To: <819ecbcd-18e3-0f6b-6121-67cb363df440@collabora.com>
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.
next prev parent reply other threads:[~2019-06-10 20:39 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-08 11:30 [PATCH v3 0/4] backlight: pwm_bl: support linear interpolation and brightness to human eye Enric Balletbo i Serra
2018-02-08 11:30 ` [PATCH v3 1/4] backlight: pwm_bl: linear interpolation between brightness-levels Enric Balletbo i Serra
2018-04-06 15:46 ` Daniel Thompson
2018-02-08 11:30 ` [PATCH v3 2/4] dt-bindings: pwm-backlight: add a num-interpolation-steps property Enric Balletbo i Serra
2018-02-18 22:49 ` Rob Herring
2018-02-08 11:30 ` [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye Enric Balletbo i Serra
2018-04-06 15:51 ` Daniel Thompson
2019-06-07 22:19 ` Matthias Kaehlcke
2019-06-08 21:02 ` Pavel Machek
2019-06-10 10:00 ` Enric Balletbo i Serra
2019-06-10 20:39 ` Matthias Kaehlcke [this message]
2019-06-10 21:02 ` Enric Balletbo i Serra
2019-06-10 21:54 ` Matthias Kaehlcke
2019-06-10 20:52 ` Matthias Kaehlcke
2019-06-11 10:49 ` Daniel Thompson
2019-06-11 16:55 ` Brian Norris
2019-06-11 22:30 ` Matthias Kaehlcke
2019-06-12 9:54 ` Pavel Machek
2019-06-12 11:03 ` Daniel Thompson
2019-06-12 19:26 ` Matthias Kaehlcke
2019-06-12 19:47 ` Daniel Thompson
2019-06-12 21:59 ` Matthias Kaehlcke
2019-06-17 13:01 ` Pavel Machek
2019-06-17 20:03 ` Matthias Kaehlcke
2018-02-08 11:30 ` [PATCH v3 4/4] dt-bindings: pwm-backlight: move brightness-levels to optional Enric Balletbo i Serra
2018-03-19 16:04 ` [PATCH v3 0/4] backlight: pwm_bl: support linear interpolation and brightness to human eye Enric Balletbo Serra
2018-03-20 11:22 ` Daniel Thompson
2018-03-20 12:13 ` Enric Balletbo Serra
2018-04-06 15:54 ` Daniel Thompson
2018-04-09 8:17 ` Lee Jones
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190610203928.GA137143@google.com \
--to=mka@chromium.org \
--cc=amstan@google.com \
--cc=briannorris@google.com \
--cc=daniel.thompson@linaro.org \
--cc=devicetree@vger.kernel.org \
--cc=dianders@google.com \
--cc=enric.balletbo@collabora.com \
--cc=groeck@google.com \
--cc=jacek.anaszewski@gmail.com \
--cc=jingoohan1@gmail.com \
--cc=kernel@collabora.com \
--cc=lee.jones@linaro.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-leds@vger.kernel.org \
--cc=pavel@ucw.cz \
--cc=robh+dt@kernel.org \
--cc=rpurdie@rpsys.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).