linux-kernel - Re: [PATCH v2 2/3] pwm: bcm2835: allow pwm driver to be used in atomic context

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Message-ID: <20231013175140.dzlz4grrgngoyxbc@pengutronix.de>
Date:   Fri, 13 Oct 2023 19:51:40 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Alexander Stein <alexander.stein@...tq-group.com>
Cc:     Sean Young <sean@...s.org>, linux-media@...r.kernel.org,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Stefan Wahren <wahrenst@....net>, linux-pwm@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] pwm: bcm2835: allow pwm driver to be used in
 atomic context

Hello,

On Fri, Oct 13, 2023 at 01:13:50PM +0200, Alexander Stein wrote:
> Am Freitag, 13. Oktober 2023, 13:04:48 CEST schrieb Stefan Wahren:
> > Am 13.10.23 um 12:46 schrieb Sean Young:
> > > clk_get_rate() may do a mutex lock. Since the clock rate cannot change on
> > > an rpi, simply fetch it once.
> > 
> > does it mean you checked all possible SoCs (BCM2835, BCM2836, BCM2837,
> > BCM2711, BCM2712) for this change?
> > 
> > Is it impossible that the real clock can never be influenced by turbo
> > mode like SPI?
> 
> Assuming the clock can change, which I would, then a clock notifier seems 
> appropriate. See [1] for an example.

I'm not a fan. If the clock changes, the output also changes. With a
clock notifier you can soften the issue and reconfigure to something
similar as the original wave form, but a glitch happens for sure.

I already toyed with the thought to add clk_rate_exclusive_get() to all
PWM drivers, but didn't come around it yet.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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