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

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Date:	Mon, 26 Sep 2011 18:30:02 -0500
From:	Rob Herring <robherring2@...il.com>
To:	"Turquette, Mike" <mturquette@...com>
CC:	Jamie Iles <jamie@...ieiles.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 09/26/2011 05:37 PM, Turquette, Mike wrote:
> 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

By definition of simple gated clock, the other bits have to be for
another clock. The restriction is that all the other bits can only be
clock gate bits.

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

Perhaps it is rare enough that it is not worth it use generic code in
this case. If so, the documentation should be clear about this
constraint. It is not something anyone will have hit before because
everyone used a single global lock. Now with the api being split between
2 locks, this adds a new complexity.

I think the simple gated clock code should be usable for any clock
controlled by a single bit in a 32-bit register independent of other
things in that register.

One example is MX1 in (mach-imx/clock-imx1.c). The CSCR register has
single bit enable for clk16m while hclk and clk48m have dividers in that
register.

Rob
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