Re: [RFC] firmware: arm_scmi: clock: add fixed clock attribute support

[Cristian Marussi <cristian.marussi@arm.com>]

On Tue, Oct 10, 2023 at 10:12:23AM +0100, Sudeep Holla wrote:
> On Tue, Oct 10, 2023 at 10:29:11AM +0800, Peng Fan (OSS) wrote:
> > From: Peng Fan <peng.fan@nxp.com>
> > 
> > There are clocks:
> >  system critical, not allow linux to disable, change rate
> >  allow linux to get rate, because some periphals will use the frequency
> >  to configure periphals.
> > 
> >  So introduce an attribute to indicated FIXED clock
> > 
> > Signed-off-by: Peng Fan <peng.fan@nxp.com>
> > ---
> >  drivers/clk/clk-scmi.c            | 6 ++++++
> >  drivers/firmware/arm_scmi/clock.c | 5 ++++-
> >  include/linux/scmi_protocol.h     | 1 +
> >  3 files changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/clk/clk-scmi.c b/drivers/clk/clk-scmi.c
> > index 8cbe24789c24..a539a35bd45a 100644
> > --- a/drivers/clk/clk-scmi.c
> > +++ b/drivers/clk/clk-scmi.c
> > @@ -182,6 +182,10 @@ static const struct clk_ops scmi_clk_ops = {
> >  	.determine_rate = scmi_clk_determine_rate,
> >  };
> >  
> > +static const struct clk_ops scmi_fixed_rate_clk_ops = {
> > +	.recalc_rate = scmi_clk_recalc_rate,
> > +};
> > +
> >  static const struct clk_ops scmi_atomic_clk_ops = {
> >  	.recalc_rate = scmi_clk_recalc_rate,
> >  	.round_rate = scmi_clk_round_rate,
> > @@ -293,6 +297,8 @@ static int scmi_clocks_probe(struct scmi_device *sdev)
> >  		if (is_atomic &&
> >  		    sclk->info->enable_latency <= atomic_threshold)
> >  			scmi_ops = &scmi_atomic_clk_ops;
> > +		else if (sclk->info->rate_fixed)
> > +			scmi_ops = &scmi_fixed_rate_clk_ops;
> >  		else
> >  			scmi_ops = &scmi_clk_ops;
> >  
> > diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
> > index ddaef34cd88b..8c52db539e54 100644
> > --- a/drivers/firmware/arm_scmi/clock.c
> > +++ b/drivers/firmware/arm_scmi/clock.c
> > @@ -46,6 +46,7 @@ struct scmi_msg_resp_clock_attributes {
> >  #define SUPPORTS_RATE_CHANGE_REQUESTED_NOTIF(x)	((x) & BIT(30))
> >  #define SUPPORTS_EXTENDED_NAMES(x)		((x) & BIT(29))
> >  #define SUPPORTS_PARENT_CLOCK(x)		((x) & BIT(28))
> > +#define SUPPORTS_FIXED_RATE_CLOCK(x)		((x) & BIT(27))
> 
> I don't see this in the specification, am I missing something ?
> 
> And why do we need it. Can't this be discrete clock with only one clock
> rate ? Or step clock with both lowest and highest the same and step being 0.
> At-least I don't see the need to change the spec for this and hence no need
> to assign any attribute bit-field to represent the same.
> 

No this is not in the spec, it would require a spec change.

My understanding is that they have clocks that CAN have more than one rate BUT
such clock cannot be changed by Linux, only other agents can
enable/disable/set_rate BUT they still want to be able to query the
current rate for configuration purposes.

Thanks,
Cristian