common clock framwork: clk_set_rate issue

[Chao Xie]

On Tue, Dec 18, 2012 at 3:47 PM, Sascha Hauer <s.hauer at pengutronix.de> wrote:
> On Tue, Dec 18, 2012 at 10:19:21AM +0800, Chao Xie wrote:
>> On Tue, Dec 18, 2012 at 4:19 AM, Sascha Hauer <s.hauer at pengutronix.de> wrote:
>> > On Thu, Dec 06, 2012 at 10:52:03AM +0800, Chao Xie wrote:
>> >> hi
>> >> When develop the clk drivers for SOCs based on common clock framework.
>> >> I met a issue.
>> >> For example there is a uart device, it's function clock comes from a
>> >> divider, and the divider's parent is a mux. It means
>> >>
>> >> MUX --> DIV --> UART
>> >>
>> >> As we know that UART can work at low baudrate for a terminal, while it
>> >> can also connect to GPS module which needs a high rate. So the MUX
>> >> will provide two clock source, a low clock rate and high clock rate.
>> >>
>> >> The MUX clk driver clk-mux.c does not implement a ->round_rate callbacks.
>> >> It means that when uart driver is used for a GPS and it want to change
>> >> it clock, the driver will call clk_set_rate(); clk_set_rate will loop
>> >> upward to DIV, and DIV will try to set its divider, and it need loop
>> >> upward to MUX.
>> >> In fact the current clk drivers have some issue.
>> >> MUX clk driver should provide the round_rate callback, it then can
>> >> provide a new_rate. It means that in clk_calc_subtree MUX can switch
>> >> the clock source.
>> >
>> > It's not that simple. The input clocks to a mux may not only differ in
>> > their rate but can also have other different properties, like for
>> > example one input may be always present whereas another input only runs
>> > when the CPU is in run mode.
>> >
>> > It may be a possibility to add a flag to the mux to explicitely
>> > allow reparenting on a rate change.
>> >
>> There is already a flag to do it.
>> CLK_SET_RATE_PARENT
>
> That flag has another meaning. It means that a clock is allowed to
> change the parents rate when a rate change is requested. What I meant
> is a flag that allows a mux to change its parent when a rate change is
> requested.
>
another flag can help it. what i mean is that i can clear the flag
CLK_SET_RATE_PARENT of MUX's children, so when clock changing will not
impact the MUX.

>> if the mux does not want to changes the input for clk_set_rate called
>> by its child, it can clear this flag.
>> The question is whether we need add the round_rate/recalc_rate for MUX
>> type of clock? Is there any special issue about it that why current
>> MUX implementation does not have these callbacks?
>
> They currently do not need these callbacks. When a clock does not have
> round_rate propagates up to the parent if CLK_SET_RATE_PARENT is set or
> it returns the parents rate if this flag is not set. The situation with
> set_rate is similar.
>
So it means that for a uart clock  that get its clock from
MUX --> DIV --> UART
when uart want to change its rate, the driver has to know the topology
of clock tree. It can not directly clk_set_rate(uart), because it only
change the DIV settings, and if we want to change the MUX input, we
have to invoke clk_set_parent(MUX). So the uart driver need to know
the clock tree topology, and it is not good for sharing a driver
between mutiple SOCs that has same UART controller IP.
That is what i am confused about.

> Sascha
>
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