linux-kernel - Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

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Message-ID: <CAGngYiXxfz7rtsw4zSj5QX7Lj7hvnoESqyUE_2__=oDaRmGGJQ@mail.gmail.com>
Date:   Fri, 29 Jan 2021 16:24:06 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

Hi Clemens,

On Fri, Jan 29, 2021 at 3:37 PM Clemens Gruber
<clemens.gruber@...ruber.com> wrote:
>
> Is the driver really responsible for bootloaders that program the chip
> with invalid values?

No, but it's responsible for correcting invalid values. Otherwise the driver
doesn't work.

> The chip comes out of PoR with sane default values. If the bootloader of
> a user messes them up, isn't that a bootloader problem instead of a
> Linux kernel driver problem?

Invalid values are only half the problem. The other half is that two valid
values might produce the same output, e.g.:

LEN_ON = 409, LED_OFF = 1228 and
LED_ON = 419, LED_OFF = 1238
produce the same result. you can't see the difference between the two
when scoping the channel. there are probably more ways to do this,
some might surprise us. It's a tricky chip.