linux-kernel - Re: [PATCH v3 3/5] soc: rockchip: add reboot notifier driver

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Date:	Thu, 21 Jan 2016 17:20:37 +0100
From:	Thierry Reding <treding@...dia.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	Andy Yan <andy.yan@...k-chips.com>, <heiko@...ech.de>,
	<linux-kernel@...r.kernel.org>, <mark.rutland@....com>,
	<devicetree@...r.kernel.org>, <khilman@...aro.org>,
	<linux@....linux.org.uk>, <pawel.moll@....com>,
	<ijc+devicetree@...lion.org.uk>, <benchan@...gle.com>,
	<sjg@...omium.org>, <linux-rockchip@...ts.infradead.org>,
	<robh+dt@...nel.org>, <galak@...eaurora.org>, <wxt@...k-chips.com>,
	<john.stultz@...aro.org>
Subject: Re: [PATCH v3 3/5] soc: rockchip: add reboot notifier driver

On Mon, Dec 28, 2015 at 04:35:46PM +0100, Arnd Bergmann wrote:
> On Monday 28 December 2015 10:20:56 Thierry Reding wrote:
> > > > > 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

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