linux-kernel - Re: [PATCH] clocksource: sh_cmt: Fix wrong setting if don't request IRQ for clock source channel

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Date:   Wed, 11 Aug 2021 16:06:06 +0200
From:   Wolfram Sang <wsa+renesas@...g-engineering.com>
To:     Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Phong Hoang <phong.hoang.wz@...esas.com>
Subject: Re: [PATCH] clocksource: sh_cmt: Fix wrong setting if don't request
 IRQ for clock source channel

Hi Daniel,

> > > If CMT instance has at least two channels, one channel will be used
> > > as a clock source and another one used as a clock event device.
> > > In that case, IRQ is not requested for clock source channel so
> > > sh_cmt_clock_event_program_verify() might work incorrectly.
> > > Besides, when a channel is only used for clock source, don't need to
> > > re-set the next match_value since it should be maximum timeout as
> > > it still is.
> > > 
> > > On the other hand, due to no IRQ, total_cycles is not counted up
> > > when reaches compare match time (timer counter resets to zero),
> > > so sh_cmt_clocksource_read() returns unexpected value.
> > > Therefore, use 64-bit clocksoure's mask for 32-bit or 16-bit variants
> > > will also lead to wrong delta calculation. Hence, this mask should
> > > correspond to timer counter width, and above function just returns
> > > the raw value of timer counter register.
> > 
> > I'm not getting the 'ch->cmt->num_channels == 1' change, can you explain?
> 
> My understanding is that if more then one channel is available the 
> channel used as clocksource is used without an interrupt. This was not 
> addressed in the patches listed as fixes. This patch fixes this multi 
> channel use-case while still retaining the old behavior for for the case 
> where only one channel is available (ch->cmt->num_channels == 
> 1).

Did Niklas answer help you?

Happy hacking,

   Wolfram


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