RE: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM SMC/HVC mailbox

[Peng Fan]

> Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc for the ARM
> SMC/HVC mailbox
> 
> On Fri, 30 Aug 2019 03:12:29 -0500
> Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
> 
> Hi,
> 
> > On Fri, Aug 30, 2019 at 3:07 AM Peng Fan <peng.fan@xxxxxxx> wrote:
> > >
> > > > Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding doc
> > > > for the ARM SMC/HVC mailbox
> > > >
> > > > On Fri, Aug 30, 2019 at 2:37 AM Peng Fan <peng.fan@xxxxxxx> wrote:
> > > > >
> > > > > Hi Jassi,
> > > > >
> > > > > > Subject: Re: [PATCH v5 1/2] dt-bindings: mailbox: add binding
> > > > > > doc for the ARM SMC/HVC mailbox
> > > > > >
> > > > > > On Fri, Aug 30, 2019 at 1:28 AM Peng Fan <peng.fan@xxxxxxx>
> wrote:
> > > > > >
> > > > > > > > > +examples:
> > > > > > > > > +  - |
> > > > > > > > > +    sram@910000 {
> > > > > > > > > +      compatible = "mmio-sram";
> > > > > > > > > +      reg = <0x0 0x93f000 0x0 0x1000>;
> > > > > > > > > +      #address-cells = <1>;
> > > > > > > > > +      #size-cells = <1>;
> > > > > > > > > +      ranges = <0 0x0 0x93f000 0x1000>;
> > > > > > > > > +
> > > > > > > > > +      cpu_scp_lpri: scp-shmem@0 {
> > > > > > > > > +        compatible = "arm,scmi-shmem";
> > > > > > > > > +        reg = <0x0 0x200>;
> > > > > > > > > +      };
> > > > > > > > > +
> > > > > > > > > +      cpu_scp_hpri: scp-shmem@200 {
> > > > > > > > > +        compatible = "arm,scmi-shmem";
> > > > > > > > > +        reg = <0x200 0x200>;
> > > > > > > > > +      };
> > > > > > > > > +    };
> > > > > > > > > +
> > > > > > > > > +    firmware {
> > > > > > > > > +      smc_mbox: mailbox {
> > > > > > > > > +        #mbox-cells = <1>;
> > > > > > > > > +        compatible = "arm,smc-mbox";
> > > > > > > > > +        method = "smc";
> > > > > > > > > +        arm,num-chans = <0x2>;
> > > > > > > > > +        transports = "mem";
> > > > > > > > > +        /* Optional */
> > > > > > > > > +        arm,func-ids = <0xc20000fe>, <0xc20000ff>;
> > > > > > > > >
> > > > > > > > SMC/HVC is synchronously(block) running in "secure mode",
> > > > > > > > i.e, there can only be one instance running platform wide. Right?
> > > > > > >
> > > > > > > I think there could be channel for TEE, and channel for Linux.
> > > > > > > For virtualization case, there could be dedicated channel for each
> VM.
> > > > > > >
> > > > > > I am talking from Linux pov. Functions 0xfe and 0xff above,
> > > > > > can't both be active at the same time, right?
> > > > >
> > > > > If I get your point correctly,
> > > > > On UP, both could not be active. On SMP, tx/rx could be both
> > > > > active, anyway this depends on secure firmware and Linux firmware
> design.
> > > > >
> > > > > Do you have any suggestions about arm,func-ids here?
> > > > >
> > > > I was thinking if this is just an instruction, why can't each
> > > > channel be represented as a controller, i.e, have exactly one func-id per
> controller node.
> > > > Define as many controllers as you need channels ?
> > >
> > > I am ok, this could make driver code simpler. Something as below?
> > >
> > >     smc_tx_mbox: tx_mbox {
> > >       #mbox-cells = <0>;
> > >       compatible = "arm,smc-mbox";
> > >       method = "smc";
> > >       transports = "mem";
> > >       arm,func-id = <0xc20000fe>;
> > >     };
> > >
> > >     smc_rx_mbox: rx_mbox {
> > >       #mbox-cells = <0>;
> > >       compatible = "arm,smc-mbox";
> > >       method = "smc";
> > >       transports = "mem";
> > >       arm,func-id = <0xc20000ff>;
> > >     };
> > >
> > >     firmware {
> > >       scmi {
> > >         compatible = "arm,scmi";
> > >         mboxes = <&smc_tx_mbox>, <&smc_rx_mbox 1>;
> > >         mbox-names = "tx", "rx";
> > >         shmem = <&cpu_scp_lpri>, <&cpu_scp_hpri>;
> > >       };
> > >     };
> > >
> > Yes, the channel part is good.
> > But I am not convinced by the need to have SCMI specific "transport" mode.
> 
> Why would this be SCMI specific and what is the problem with having this
> property?
> By the very nature of the SMC/HVC call you would expect to also pass
> parameters in registers. However this limits the amount of data you can push,
> so the option of reverting to a memory based payload sounds very
> reasonable.
> On the other hand *just* using memory complicates things, in case you have a
> very simple protocol. You would need a memory region shared between
> firmware and OS, which is not always easily possible on every platform. Also
> this doesn't scale easily with multiple mailboxes and channels. Passing
> parameters via registers is also naturally consistent, as there would be no
> races and no need for synchronisation with other cores or other users of the
> mailbox.
> 
> So I clearly see the benefit of specifying *both* ways of payload transport.
> Given that this driver should be protocol agnostic, it makes a lot of sense to
> introduce both methods *now*, so in the future users can just use the register
> method, without extending the binding in a incompatible way later (earlier
> kernels would have the driver, but wouldn't know how to deal with this
> parameter).

Andre, thanks for your explanation.
Jassi, are you ok that this property "transport" is kept in V6?

Thanks,
Peng.
> 
> Cheers,
> Andre.