linux-kernel - Re: [PATCH v10 2/2] leds: Add driver for Qualcomm LPG

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Message-ID: <Yfr9+jvGIyB2ynMS@ripper>
Date:   Wed, 2 Feb 2022 13:56:10 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
        Lee Jones <lee.jones@...aro.org>, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-pwm@...r.kernel.org,
        Yassine Oudjana <y.oudjana@...tonmail.com>,
        Luca Weiss <luca@...tu.xyz>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>
Subject: Re: [PATCH v10 2/2] leds: Add driver for Qualcomm LPG

On Wed 02 Feb 03:03 PST 2022, Marijn Suijten wrote:

> On 2022-01-28 18:50:42, Bjorn Andersson wrote:
> > On Wed 27 Oct 16:19 CDT 2021, Marijn Suijten wrote:
> > 
> > > Hi Bjorn,
> > > 
> > > On 2021-10-22 10:25:35, Bjorn Andersson wrote:
> > > > On Sat 09 Oct 21:39 PDT 2021, Bjorn Andersson wrote:
> > > > 
> > > > > The Light Pulse Generator (LPG) is a PWM-block found in a wide range of
> > > > > PMICs from Qualcomm. These PMICs typically comes with 1-8 LPG instances,
> > > > > with their output being routed to various other components, such as
> > > > > current sinks or GPIOs.
> > > > > 
> > > > > Each LPG instance can operate on fixed parameters or based on a shared
> > > > > lookup-table, altering the duty cycle over time. This provides the means
> > > > > for hardware assisted transitions of LED brightness.
> > > > > 
> > > > > A typical use case for the fixed parameter mode is to drive a PWM
> > > > > backlight control signal, the driver therefor allows each LPG instance
> > > > > to be exposed to the kernel either through the LED framework or the PWM
> > > > > framework.
> > > > > 
> > > > > A typical use case for the LED configuration is to drive RGB LEDs in
> > > > > smartphones etc, for which the driver support multiple channels to be
> > > > > ganged up to a MULTICOLOR LED. In this configuration the pattern
> > > > > generators will be synchronized, to allow for multi-color patterns.
> > > > > 
> > > > > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> > > > > ---
> > > > 
> > > > Any feedback on this?
> > > 
> > > I asked in #linux-msm whether anything is wrong with the patterns,
> > > since my Sony Discovery (sdm630 with a pm660l) blinks way quicker on a
> > > pattern that's supposed to stay on for 1s and off for 1s:
> > > 
> > >     echo "0 1000 255 1000" > /sys/class/leds/rgb\:status/hw_pattern
> > > 
> > > It however seems to be broken in the same way on an older version now
> > > (this might be v9 or v8) which I don't remember to be the case.  Can you
> > > double-check if this is all working fine on your side?  If so, I'll have
> > > to find some time to debug it on my end.
> > > 
> > 
> > I had missed the fact that LPG_RAMP_DURATION_REG is two registers for
> > msg and lsb, for a total of 9 bits of duration. So what you saw was
> > probably ticking at 232ms.
> > 
> > Note though that the pattern uses the last time as "high pause", so I
> > expect that you should have seen 232 ms of off, followed by 464ms of
> > light.
> 
> Visual inspection seems to confirm those numbers indeed!
> 
> > I've fixed this for v11, both rejecting invalid input and writing out
> > all 9 bits.
> 
> Doesn't that 512ms limit, together with using only the last value for
> hi_pause (and not the first value for lo_pause) force users to write
> patterns in a certain way which is not easily conveyed to the caller
> except by reading the comment in the driver?  I'd guess lo_pause can be
> used even if not in ping-pong mode, it should just hold at the first
> value for the given duration?
> 
> (That said hw_pattern is anyway already riddled with device-specific
> information, such as only having one `delta_t` which functions as the
> step size for every entry, and with the change above would need to be
> sourced from another step that's not the first.)
> 

Perhaps we should clarify the single delta_t by requiring all those
delta_t to be the same, rather than ignoring their value.

I.e. we make the ping-pong pattern:

<value> <lopause+t> ... <value[N/2-1]> <t> <value[N/2]> <hipause+t> <value[N/2-1]> <t> ... <value> <t>

And for non-ping-pong:

<value> <lopause+t> <value> <t> ... <value> <t> <value> <hipause + t>


What do you think?

> Bit of a stretch, but perhaps worth noting anyway: should this be
> written in documentation somewhere, together with pattern examples and
> their desired outcome to function as testcases too?
> 

There's a comment in lpg_pattern_set() where I tried to capture this.

I don't think it's worth documenting the behavior/structure away from
the driver. But let's make sure it's captured properly there.

Regards,
Bjorn