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

[Peng Fan]

> Subject: Re: [RFC] firmware: arm_scmi: clock: add fixed clock attribute
> support
> 
> On Tue, Oct 10, 2023 at 08:08:01AM +0000, Peng Fan wrote:
> > > Subject: Re: [RFC] firmware: arm_scmi: clock: add fixed clock
> > > attribute support
> > >
> > > On Tue, Oct 10, 2023 at 10:29:11AM +0800, Peng Fan (OSS) wrote:
> > > > From: Peng Fan <peng.fan at 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
> > > >
> > >
> > > Hi,
> > >
> > > (CCed souvik.chakravarty at arm.com)
> > >
> > > so AFAIU here you are describing a clock that really is NOT fixed in
> > > general, it is just that the Linux SCMI Agent cannot touch it, but
> > > other SCMI agents on the system CAN change it and so, on one side,
> > > you keep the ability for the Linux agent to read back the current
> > > rate with
> > > recalc_rate() and remove all the Clk frameworks callbacks needed to
> > > modify its state, am I right ?
> >
> > Right.
> >
> > >
> > > In this scenario, it is really the SCMI platform fw (server) that
> > > has to implement the checks and simply DENY the requests coming from
> > > an agent that is not supposed to touch that clock, while allowing
> > > the current rate to be retrieved.
> >
> > Linux will try to enable, get rate, runtime disable the clock.
> > But the server does not allow enable/disable the clock, so the driver
> > probe will fail.
> >
> 
> Which driver probe ? I have just double checked and when clk-scmi driver is
> loaded there are a bunch of SCMI queries to the server BUT no *_SET
> command is issued during the clk-scmi probe; indeed JUNO had never had
> any issue despite having access to a bunch of CLKs which are visibile BUT not
> modifiable from Linux.

It is my setup, I2C needs CLK BUSAON(this clock is also critical for system),
SCMI server exposes this clock, allow get rate, but when linux try to enable
this clock, SCMI server reply denied, so the i2c driver will probe failure.

Regards,
Peng.

> 
> > The SCMI server could bypass enable/disable and only allow get rate,
> > But this introduces heavy RPC, so just wanna whether it is ok to
> > register fixed clock and avoid RPC.
> >
> > >
> > > JUNO/SCP is an example of how the CPUs clocks are visible to Linux
> > > BUT cannot be touched directly via Clock protocol by Linux since in
> > > the SCMI world you are supposed to use the Perf protocol instead to
> > > change the OPPs when you want to modify the performance level of the
> runnning CPU.
> > >
> > > This kind of server-side permissions checks, meant to filter access
> > > to resources based on the requesting agent, are part of the SCMI
> > > declared aim to push the responsibility of such controls out of the
> > > kernel into the platform fw in order to avoid attacks like
> > > CLOCK_SCREW by letting the SCMI firmware be the one and only final
> > > arbiter on the requests coming from the agents; you can ask teh
> > > server whatever you like as an agent but your request can be DENIED
> > > or silently ignored (in case of shared resources) at the will of the platform
> which has the final say and it is implemented in a physically distinct code-
> base.
> > >
> > > It seems to me that this patch and the possible associated SCMI
> > > specification change would give back the control to the Linux agent
> > > and could allow the implementation of an SCMI Server that does NOT
> > > perform any of these permission checks.
> > >
> > > So, IMO, while this change, on one side, could be certainly useful
> > > by removing a bunch of unused/uneeded callbacks from the CLK SCMI
> > > driver when a fixed clock is identified, it could open the door to a
> > > bad implementation like the one mentioned above which does NOT
> > > perform any agent-based permission check.
> >
> > Thanks for detailed information, let me check whether our SCMI
> > firmware could do more on the permission side. But if RPC could be
> > removed, it could save some time.
> >
> 
> Avoiding to issue SCMI requests destined to fail would be certainly good,
> even though it could lead to just quietly implementing a server with no-
> checks at all, but why these unneeded calls happen in first place ?
> 
> I have never observed anything like that in my setups.
> 
> Thanks,
> Cristian