common clock framwork: clk_set_rate issue

[Sascha Hauer]

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.
 
> 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.

Sascha

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