linux-kernel - Re: [PATCH v6 3/3] clk: basic clock hardware types

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Date:	Mon, 12 Mar 2012 17:00:57 -0500
From:	Rob Herring <robherring2@...il.com>
To:	"Turquette, Mike" <mturquette@...com>
CC:	Andrew Lunn <andrew@...n.ch>, Paul Walmsley <paul@...an.com>,
	linaro-dev@...ts.linaro.org,
	Linus Walleij <linus.walleij@...ricsson.com>,
	patches@...aro.org, Stephen Boyd <sboyd@...eaurora.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Magnus Damm <magnus.damm@...il.com>,
	linux-kernel@...r.kernel.org,
	Rob Herring <rob.herring@...xeda.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jamie Iles <jamie@...ieiles.com>,
	Russell King <linux@....linux.org.uk>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Arnd Bergman <arnd.bergmann@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 3/3] clk: basic clock hardware types

On 03/12/2012 03:58 PM, Turquette, Mike wrote:
> On Mon, Mar 12, 2012 at 1:18 PM, Rob Herring <robherring2@...il.com> wrote:
>> On 03/10/2012 01:54 AM, Mike Turquette wrote:
>>> Many platforms support simple gateable clocks, fixed-rate clocks,
>>> adjustable divider clocks and multi-parent multiplexer clocks.
>>>
>>> This patch introduces basic clock types for the above-mentioned hardware
>>> which share some common characteristics.
>>>
>>> Based on original work by Jeremy Kerr and contribution by Jamie Iles.
>>> Dividers and multiplexor clocks originally contributed by Richard Zhao &
>>> Sascha Hauer.
>>>

snip

>>> +static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
>>> +             unsigned long parent_rate)
>>> +{
>>> +     struct clk_divider *divider = to_clk_divider(hw);
>>> +     unsigned int div;
>>> +     unsigned long flags = 0;
>>> +
>>> +     if (divider->lock)
>>> +             spin_lock_irqsave(divider->lock, flags);
>>> +
>>> +     div = readl(divider->reg) >> divider->shift;
>>> +     div &= (1 << divider->width) - 1;
>>> +
>>> +     if (divider->lock)
>>> +             spin_unlock_irqrestore(divider->lock, flags);
>>
>> What are you locking against? You are only reading the register.
> 
> Hi Rob,
> 
> These register-level locks originally came in from the divider &
> multiplexer patches from Richard and Sascha and I'm sure they can give
> you more details than I.
> 
> Basically on their platform they have some 32-bits regs that have a
> lot of overlapping clock functions in them, such as enable/disable and
> adjusting a divider all in one reg.  Those operations are protected by
> different locks (enable spinlock and prepare mutex, respectively) so
> some synchronization mechanism is necessary.  On OMAP we don't use
> this since we have a billion registers that typically only map to one
> clock each.  I also wonder if having device type memory for the
> affected regions makes a this irrelevant on ARM... but that wouldn't
> matter for non-ARM architectures.  Just a thought.
> 

In fact, I pointed out that locking for a register access was needed in
an early version of this series as well. However, locking is only needed
if you are doing a read-mod-write on a field in a shared register or
reading from multiple registers. It makes no sense if you are *only*
reading a single shared register as is the case here. That's already an
atomic operation.

Rob

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