Re: [RFC PATCH] firmware: arm_scmi: skip empty CLOCK_DESCRIBE_RATES replies

[Jay Buddhabhatti]

Hi Sudeep,

On 10/5/2026 9:01 PM, Sudeep Holla wrote:
> On Mon, Oct 05, 2026 at 06:06:09PM +0530, Jay Buddhabhatti wrote:
> > Hi Sudeep,
> >
> > Thanks for the review. Please find my response inline.
> >
> > On 10/1/2026 8:14 PM, Sudeep Holla wrote:
> > > On Thu, Oct 01, 2026 at 04:45:38AM -0700, Jay Buddhabhatti wrote:
> > > > Some platforms advertise reserved or uninstantiated clock IDs that still
> > > > succeed CLOCK_DESCRIBE_RATES with zero rates. After dynamic rate
> > > > allocation, kcalloc(0) returns ZERO_SIZE_PTR and protocol init then
> > > > dereferences rates[0], which panics.
> > > >
> > > > Do not allocate or index the rate array when the firmware reports an
> > > > empty list, so unused IDs are skipped instead of taking down the SCMI
> > > > clock provider.
> > > >
> > > > Fixes: 62ba967595e0 ("firmware: arm_scmi: Make clock rates allocation dynamic")
> > > > Signed-off-by: Jay Buddhabhatti <jay.buddhabhatti@xxxxxxx>
> > > > ---
> > > > The SCMI server is the source of this zero rate and successful response
> > > > and it should be fixed in SCMI server. This defensive check in Linux is
> > > > still useful because firmware responses must be validated before
> > > > de-referencing dynamically allocated data, The panic is a Linux
> > > > regression introduced by dynamic rate allocation; previous fixed array
> > > > tolerated the same response and other SCMI implementations could return
> > > > the same unexpected response.
> > > > ---
> > > >    drivers/firmware/arm_scmi/clock.c | 18 ++++++++++++++++++
> > > >    1 file changed, 18 insertions(+)
> > > >
> > > > diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
> > > > index 0278705d809e..8934a95527e2 100644
> > > > --- a/drivers/firmware/arm_scmi/clock.c
> > > > +++ b/drivers/firmware/arm_scmi/clock.c
> > > > @@ -8,6 +8,7 @@
> > > >    #include <linux/math64.h>
> > > >    #include <linux/module.h>
> > > >    #include <linux/limits.h>
> > > > +#include <linux/slab.h>
> > > >    #include <linux/sort.h>
> > > >    #include "protocols.h"
> > > > @@ -484,6 +485,13 @@ iter_clk_describe_update_state(struct scmi_iterator_state *st,
> > > >    	if (!st->max_resources) {
> > > >    		unsigned int tot_rates = st->num_returned + st->num_remaining;
> > > > +		/*
> > > > +		 * Unused/reserved clock IDs return 0 rates. kmalloc(0)
> > > > +		 * returns ZERO_SIZE_PTR and must not be dereferenced.
> > > > +		 */
> > > > +		if (!tot_rates)
> > > > +			return 0;
> > > > +
> > > >    		p->clkd->r.rates = devm_kcalloc(p->dev, tot_rates,
> > > >    						sizeof(*p->clkd->r.rates), GFP_KERNEL);
> > > >    		if (!p->clkd->r.rates)
> > > > @@ -505,6 +513,9 @@ iter_clk_describe_process_response(const struct scmi_protocol_handle *ph,
> > > >    	struct scmi_clk_ipriv *p = priv;
> > > >    	const struct scmi_msg_resp_clock_describe_rates *r = response;
> > > > +	if (ZERO_OR_NULL_PTR(p->clkd->r.rates))
> > > > +		return -EPROTO;
> > > > +
> > > >    	p->clkd->r.rates[p->clkd->r.num_rates] = RATE_TO_U64(r->rate[st->loop_idx]);
> > > >    	/* Count only effectively discovered rates */
> > > > @@ -622,6 +633,13 @@ scmi_clock_describe_rates_get(const struct scmi_protocol_handle *ph,
> > > >    	if (ret)
> > > >    		return ret;
> > > > +	/*
> > > > +	 * Some platforms expose reserved clock IDs with an empty
> > > > +	 * CLOCK_DESCRIBE_RATES reply. Do not dereference rates[].
> > > > +	 */
> > > > +	if (!clkd->r.num_rates || ZERO_OR_NULL_PTR(clkd->r.rates))
> > > > +		return 0;
> > > > +
> > >
> > > I expect the Clock rate control bit to be unset in the permissions for
> > > these clock, else it may be dangerous to do this. Please add that check.
> >
> > I will add that check in new version. If the Clock rate control bit is set,
> > Linux clears its local copy by setting rate_ctrl_forbidden. The clock
> > remains registered, but clk-scmi does not install set_rate and
> > scmi_clock_rate_set() returns -EACCES. This avoids exposing rate changes
> > when no valid rates were described.
>
> You did mention this is issue in the firmware. If the generic solution(once
> agreed upon) doesn't work on your platform, then you need to fix it with
> a quirk I am afraid. I will let you propose the patch and take it from there.

Agreed, Linux should not override the platform permissions. I'll drop
that in v2. If CLOCK_DESCRIBE_RATES returns no rates, v2 accepts the
reply only if:

 - the clock has no name. clk-scmi already skips unnamed clocks via
   scmi_clock_info_get(), so these are never registered, or
 - the platform reports rate control as not allowed. The clock is kept
   for enable/disable and no set_rate is installed.

Otherwise it logs an error and invalidates the clock, so that it is not
registered with an undefined rate range.

The de-reference fixes from v1 stay as they are, since they
avoid the panic in all cases.

I checked our platform SCMI server: all the reserved IDs that return an
empty rate list also return an empty name, so they were never exposed
by clk-scmi. With v2 they are skipped silently, and every clock that is
exposed has a valid rate list, so I don't think a quirk is needed.

Does this sound reasonable to you? If so, I'll post v2.

Thanks,
Jay
>