On Mon, Dec 28, 2015 at 04:35:46PM +0100, Arnd Bergmann wrote:

On Monday 28 December 2015 10:20:56 Thierry Reding wrote:

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HTC apparently uses a separate RAM area to pass the reboot reason,
and they have a driver to store that, which is separate from the
driver that they use for actually rebooting the machine.

I wasn't very clear, but the PMC_SCRATCH0 register is used to store the
reset reason. It supports the recovery mode, which I think is really an
Android thing, "bootloader" will typically cause the bootloader not to
boot anything, and "forced-recovery" will go into a recovery mode that
is used to bootstrap the device (usually by uploading a "miniloader"
that initializes RAM, downloads a bootloader for booting or flashing an
operating system, ...).

The write that resets the SoC is to a different register.

So is this scratch register interpreted by some maskrom code, or by code that
can be provided by the OEM?

My understanding is that its interpreted both by what's called BootROM
on Tegra (I guess that's what you call "maskrom code") and the system's
bootloader. The BootROM cannot typically be replaced by the OEM, but it
is quite typical for the bootloader to differ between devices.

Ok, so not maskrom (which would not be OEM specific, but hardcoded for
the chip) but rather some form of PROM. This means we can only guess
that all OEMs use the same protocol but in theory someone could have
implemented an incompatible BootROM, but it's also possible that HTC
just ignore the register entirely and implement the same thing separately.

I wasn't being clear, the BootROM is hardcoded for the chip, I'm not
aware of a way to replace it once the chip's taped out.

Thierry