Raspberry Pi power control

Looking in RiscOS/Sources/HAL/HAL_BCM2835/s/Top I find:

        ; Enable USB power
        ; Note - may need changing to enable other devices in future
        ; Looks like we need to write the logical OR of all the devices we want enabled
        MOV     r1, #0
        LDR     r0,=(16:SHL:MB_Pwr_USB)+MB_Chan_Pwr
        BL      HAL_SendHostMessage

This appears to be using the Videocore power management mailbox 0, which is undocumented, rather than the property interface mailbox 8. There’s an equivalent property message for SetPowerState (0×28001).

Any particular reason it is implemented this way rather than using the official interface?

Any particular reason it is implemented this way rather than using the official interface?

That code has actually been there since the very beginning (albeit in a slightly different form).

https://gitlab.riscosopen.org/RiscOS/Sources/HAL/HAL_BCM2835/-/blob/bfa96cd9f1f285e7333ba486fc4a0143a9194056/s/Top#L285

There’s a good chance that it predates the creation of the mailbox property interface. And since it still works, we haven’t had any reason to change it to use the newer interface.

I have implemented software power control for the Pi. I have used a method which is compatible with the Witty Pi (which removes power directly if GPIO 4 is taken low).

During a reset GPIO 4 goes to a high impedance state. Prior to a reset GPIO 4=HIGH is used to suppress a power off request.

A power off request (either from the ‘off’ pushbutton or from a low battery voltage state) causes GPIO 26 to be taken low.

So .. if GPIO 26 is high then this indicates that a reset will leave power on, whereas if GPIO 26 is low then a reset will remove power as soon as the rom is reset. So my method does not offer a way to remove power directly unless a shutdown request (e.g. by push button) has already been made. So I can’t offer an option ‘remove power’ or ‘reset’ but it would be nice to alter the message that says ‘The computer is now ready to be switched off’ to make the option either ‘Power off’ or ‘Restart’ depending on the status of GPIO 26. Is there a way to do this?

Thanks Jeffrey. I might have a go at reworking it to use the mailbox property interface, to bring it into line with the rest. Obviously careful testing required!

Is there any chance of old RPis running RISC OS out there with firmware that doesn’t support the Videocore mailbox property interface?

ISTR it arriving some time in 2012. So they would have to be very old Pi 1s, which debuted February 2012. Since the firmware is on the SD card it’s simple to update at the same time as the ROM image – I don’t know what the position is for supporting ancient firmware?

git blame says the documentation of the Set Power State property was last edited June 2012, so it’s been there for a long time.

So I can’t offer an option ‘remove power’ or ‘reset’ but it would be nice to alter the message that says ‘The computer is now ready to be switched off’ to make the option either ‘Power off’ or ‘Restart’ depending on the status of GPIO 26. Is there a way to do this?

Not at the moment. However it should be relatively easy to add support for it – all it really needs is for someone to design & implement an API which the OS can use to talk to power control hardware.

Since the firmware is on the SD card it’s simple to update at the same time as the ROM image – I don’t know what the position is for supporting ancient firmware?

Ancient firmware isn’t supported. I’m not sure exactly when the cutoff is – I know firmware from 2019 still works.

Ancient firmware isn’t supported. I’m not sure exactly when the cutoff is – I know firmware from 2019 still works.

January 2019 is about as far back as you can go. Any further back and you run into problems such as an inability to use 256-colour screen modes