linux-kernel - Re: [PATCH v6 4/5] clk: aspeed: Register gated clocks

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Date:   Tue, 2 Jan 2018 10:16:08 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Joel Stanley <joel@....id.au>, Lee Jones <lee.jones@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Andrew Jeffery <andrew@...id.au>, Jeremy Kerr <jk@...abs.org>,
        Rick Altherr <raltherr@...gle.com>,
        Ryan Chen <ryan_chen@...eedtech.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v6 4/5] clk: aspeed: Register gated clocks

On 12/30, Benjamin Herrenschmidt wrote:
> On Tue, 2017-12-26 at 17:32 -0800, Stephen Boyd wrote:
> > > I noticed we do have a few i2c based clock drivers... how are they ever
> > > supposed to work ? i2c bus controllers are allowed to sleep and the i2c
> > > core takes mutexes...
> > 
> > We have clk_prepare()/clk_unprepare() for sleeping suckage. You
> > can use that, and i2c based clk drivers do that today.
> 
> "suckage" ? Hehe ... the suckage should rather be stuff that cannot
> sleep. Arbitrary latencies and jitter caused by too much code wanting
> to be "atomic" when unnecessary are a bad thing.

Heh. Of course.

> 
> In the case of clocks like the aspeed where we have to wait for a
> rather long stabilization delay, way too long to legitimately do a non-
> sleepable delay with a lock held, do we need to do everything in
> prepare() then ?
> 

Yes. If we have to wait a long time in the enable path it makes
sense to move it to the prepare path instead, if possible. That
way we avoid holding a spinlock for a long time.

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