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

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Message-ID: <CAJOA=zMA8q-acr=BGWrpC9iLSR3E8Y0ybH4sZ=FOn54zQ7TJKw@mail.gmail.com>
Date:	Mon, 26 Sep 2011 15:37:41 -0700
From:	"Turquette, Mike" <mturquette@...com>
To:	Jamie Iles <jamie@...ieiles.com>
Cc:	Rob Herring <robherring2@...il.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 12:37 PM, Jamie Iles <jamie@...ieiles.com> wrote:
> 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 ;-))

I think it is a safe assumption that if a register controls both
enable/disable and some programmable divider then,

1) those controls are probably for the same clock
2) that clock won't be using the cookie-cutter gated-clock
implementation anyways

Rob, do you feel these assumptions are OK and locking can remain the
same in this patch?

Regards,
Mike

> Jamie
>
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