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

From: Saravana Kannan
Date: Tue Oct 04 2011 - 21:41:38 EST

On 09/26/2011 04:30 PM, Rob Herring wrote:
On 09/26/2011 05:37 PM, Turquette, Mike wrote:
On Mon, Sep 26, 2011 at 12:37 PM, Jamie Iles<jamie@xxxxxxxxxxxxx> 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

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 kinda agree with Rob on this. There are very few, if any, such simple clocks on MSM chips. It's very easy to a SoC clock developer to accidentally use these simple clocks without realizing the point that Rob brings up.

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.

To take care of the scenario Rob bring up, the prepare/unprepare and enable/disable code will have to grab a per-tree register-lock before accessing any registers. The prepare/unprepare code should obviously be written to hold this register-lock for as small of a duration as possible. For example, if the prepare code is doing voltage increase, the register-lock should be grabber _after_ the voltage is increased. At least, this is approximately how the MSM clock code can be mapped onto this generic framework.

I think we should just go ahead and implement the per-tree register lock so that the generic clock implementations are more useful. The lock will really be held only for a very short time and hence shouldn't matter that there is a single lock for all the clocks in a tree.


Did you get a chance to send out your patches with support for per-tree locking? I would really like to see that as part of the first patch series that gets pulled in.


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