linux-kernel - Re: [PATCH v2 4/7] clk: Add simple gated clock

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Date:	Mon, 26 Sep 2011 20:37:33 +0100
From:	Jamie Iles <jamie@...ieiles.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Jamie Iles <jamie@...ieiles.com>,
	Mike Turquette <mturquette@...com>,
	linux-kernel@...r.kernel.org, paul@...an.com,
	linaro-dev@...ts.linaro.org, linus.walleij@...ricsson.com,
	patches@...aro.org, eric.miao@...aro.org,
	broonie@...nsource.wolfsonmicro.com, magnus.damm@...il.com,
	arnd.bergmann@...aro.org, skannan@...cinc.com,
	linux@....linux.org.uk, jeremy.kerr@...onical.com,
	tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
	sboyd@...inc.com
Subject: Re: [PATCH v2 4/7] clk: Add simple gated clock

On Mon, Sep 26, 2011 at 02:10:32PM -0500, Rob Herring wrote:
> On 09/26/2011 01:40 PM, Jamie Iles wrote:
> > On Mon, Sep 26, 2011 at 01:33:08PM -0500, Rob Herring wrote:
> >>> +static void clk_gate_set_bit(struct clk_hw *clk)
> >>> +{
> >>> +	struct clk_gate *gate = to_clk_gate(clk);
> >>> +	u32 reg;
> >>> +
> >>> +	reg = __raw_readl(gate->reg);
> >>> +	reg |= BIT(gate->bit_idx);
> >>> +	__raw_writel(reg, gate->reg);
> >>
> >> Don't these read-mod-writes need a spinlock around it?
> >>
> >> It's possible to have an enable bits and dividers in the same register.
> >> If you did a set_rate and while doing an enable/disable, there would be
> >> a problem. Also, it may be 2 different clocks in the same register, so
> >> the spinlock needs to be shared and not per clock.
> > 
> > Well the prepare lock will be held here and I believe that would be 
> > sufficient.
> 
> No, the enable spinlock is protecting enable/disable. But set_rate is
> protected by the prepare mutex. So you clearly don't need locking if you
> have a register of only 1 bit enables. If you have a register accessed
> by both enable/disable and prepare/unprepare/set_rate, then you need
> some protection.

OK fair point, but I would guess that if you had a clock like this then 
you probably wouldn't use this simple gated clock would you?  (speaking 
from my world where we have quite simple clocks ;-))

Jamie
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