linux-kernel - Re: [PATCH v5 2/2] pwm: Add clock based PWM output driver

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Date:   Sat, 19 Feb 2022 18:54:40 +0500
From:   Nikita Travkin <nikita@...n.ru>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     thierry.reding@...il.com, lee.jones@...aro.org, robh+dt@...nel.org,
        sboyd@...nel.org, krzk@...nel.org, linus.walleij@...aro.org,
        masneyb@...tation.org, sean.anderson@...o.com,
        linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH v5 2/2] pwm: Add clock based PWM output driver

Uwe Kleine-König писал(а) 19.02.2022 15:10:
> Hello,
> 
> On Sat, Feb 19, 2022 at 11:46:31AM +0500, Nikita Travkin wrote:
>> Uwe Kleine-König писал(а) 14.02.2022 23:43:
>> > On Sat, Feb 12, 2022 at 09:23:42PM +0500, Nikita Travkin wrote:
>> >> + * Limitations:
>> >> + * - Glitches are possible when new pwm state is applied.
>> >> + * - Due to the fact that exact behavior depends on the underlying
>> >> + *   clock driver, various limitations are possible.
>> >> + * - Period depends on the clock and, in general, not guaranteed.
>> >
>> > This sentence is broken.
>> >
>>
>> Here what I mean is that the clock driver might e.g. have a lookup table
>> for some rates and will only set one close to the requested ones.
>> (Extreme scenario is that only one rate is allowed in the lookup table,
>> which is a real possibility for some platforms that I think this driver
>> will be used with, the lookup may need to be changed for those clocks)
>>
>> I will reword this like:
>>
>>   Some clock drivers may only pick the closest available rate
>>   and not the exact requested one. Because of this, exact period
>>   is not guaranteed.
> 
> That there is no exact match is quite normal also for dedicated PWM
> HW blocks. So I think the second item in your list is good enough to
> cover the non-existing guaranteed for period and glitches.
> 

Oh, I will just drop the 1 (glitches) and 3 (period) then.
Thanks for explaining!

Nikita

> Best regards
> Uwe