Messages in this thread | | | Date | Tue, 18 Dec 2012 08:47:11 +0100 | From | Sascha Hauer <> | Subject | Re: common clock framwork: clk_set_rate issue |
| |
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@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
-- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
| |