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Message-ID: <80C715D4-5B8A-4FC9-AD60-106F682DC929@aspeedtech.com>
Date: Mon, 26 Jul 2021 05:14:09 +0000
From: Billy Tsai <billy_tsai@...eedtech.com>
To: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>
CC: "lee.jones@...aro.org" <lee.jones@...aro.org>,
"robh+dt@...nel.org" <robh+dt@...nel.org>,
"joel@....id.au" <joel@....id.au>,
"andrew@...id.au" <andrew@...id.au>,
"thierry.reding@...il.com" <thierry.reding@...il.com>,
"p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
BMC-SW <BMC-SW@...eedtech.com>
Subject: Re: [v9 2/2] pwm: Add Aspeed ast2600 PWM support
Hi Uwe
On 2021/7/24, 4:13 AM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:
> On Fri, Jul 23, 2021 at 04:23:23AM +0000, Billy Tsai wrote:
> > On 2021/7/23, 3:17 AM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:
> >
> > On Wed, Jul 21, 2021 at 10:52:21AM +0000, Billy Tsai wrote:
> > >> Hi Uwe,
> > >>
> > >> On 2021/7/16, 6:13 PM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:
> > >>
> > >> On Fri, Jul 16, 2021 at 09:22:22AM +0000, Billy Tsai wrote:
> > >> >> On 2021/7/16, 3:10 PM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:
> > >> >>
> > >> >> On Fri, Jul 16, 2021 at 01:48:20AM +0000, Billy Tsai wrote:
> > >> >> >> On 2021/7/15, 11:06 PM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de>> wrote:
> > >> >> >> > Another is: The PWM doesn't support duty_cycle 0, on such a request the
> > >> >> >> > PWM is disabled which results in a constant inactive level.
> > >> >> >>
> > >> >> >> > (This is correct, is it? Or does it yield a constant 0 level?)
> > >> >> >>
> > >> >> >> Our pwm can support duty_cycle 0 by unset CLK_ENABLE.
> > >> >>
> > >> >> > This has a slightly different semantic though. Some consumer might
> > >> >> > expect that the following sequence:
> > >> >>
> > >> >> > pwm_apply(mypwm, { .period = 10000, .duty_cycle = 10000, .enabled = true })
> > >> >> > pwm_apply(mypwm, { .period = 10000, .duty_cycle = 0, .enabled = true })
> > >> >> > pwm_apply(mypwm, { .period = 10000, .duty_cycle = 10000, .enabled = true })
> > >> >>
> > >> >> > results in the output being low for an integer multiple of 10 µs. This
> > >> >> > isn't given with setting CLK_ENABLE to zero, is it? (I didn't recheck,
> > >> >> > if the PWM doesn't complete periods on reconfiguration this doesn't
> > >> >> > matter much though.)
> > >> >> Thanks for the explanation.
> > >> >> Our hardware actually can only support duty from 1/256 to 256/256.
> > >> >> For this situation I can do possible solution:
> > >> >> We can though change polarity to meet this requirement. Inverse the pin and use
> > >> >> duty_cycle 100.
> > >> >> But I think this is not a good solution for this problem right?
> > >>
> > >> > If this doesn't result in more glitches that would be fine for me.
> > >> > (Assuming it is documented good enough in the code to be
> > >> > understandable.)
> > >>
> > >> > The polarity of our pwm controller will affect the duty cycle range:
> > >> > PWM_POLARITY_INVERSED : Support duty_cycle from 0% to 99%
> > >> > PWM_POLARITY_NORMAL: Support duty_cycle from 1% to 100%
> > >> > Dynamic change polarity will result in more glitches. Thus, this will become
> > >> > a trade-off between 100% and 0% duty_cycle support for user to use our pwm device.
> > >> > I will document it and send next patch.
> > >>
> > >> For handling the situation that the user want to set the duty cycle to 0%, the driver can:
> > >> 1. Just return the error.
> > >> 2. Use the minimum duty cycle value.
> > >> I don't know which solution will be the better way or others.
> > >> I would be grateful if you can give me some suggestion about this problem.
> >
> > > I thought if you disable the PWM it emits the inactive level? Then this
> > > is the best you can do if duty_cycle = 0 is requested.
> >
> > Thanks for your quick reply.
> > When duty_cycle = 0 is requested my driver currently will emit the inactive level.
> > So, the next patch I need to do is to add the comment about this?
> Not sure I got the complete picture now. The things I consider important
> are:
> - If your hardware cannot emit a 100% or 0% relative duty cycle, note
> this in the Limitations section
> - Assuming your PWM emits the inactive level when disabled (that is 0
> for PWM_POLARITY_NORMAL and 1 for PWM_POLARITY_INVERSED) this is the
> best that can be done when a 0% relative duty cycle is requested
> (assuming the hardware cannot implement that in a normal way).
Our hardware is the same as this description.
So I didn't need to add the limitations about the duty cycle, right?
Or I need to note that the duty cycle 0% is just the inactive output that doesn't have the period concept.
> I hope this answered your remaining questions.
Thanks.
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