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

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Message-ID: <20151215174234.GB24031@ulmo.nvidia.com>
Date:	Tue, 15 Dec 2015 18:42:36 +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 Tue, Dec 15, 2015 at 05:34:00PM +0100, Arnd Bergmann wrote:
> On Tuesday 15 December 2015 17:31:22 Thierry Reding wrote:
> > On Mon, Dec 14, 2015 at 12:39:44PM +0100, Arnd Bergmann wrote:
> > > On Wednesday 18 November 2015 17:56:22 Andy Yan wrote:
> > > > rockchip platform have a protocol to pass the kernel reboot
> > > > mode to bootloader by some special registers when system reboot.
> > > > By this way the bootloader can take different action according
> > > > to the different kernel reboot mode, for example, command
> > > > "reboot loader" will reboot the board to rockusb mode, this is
> > > > a very convenient way to get the board enter download mode.
> > > > 
> > > > Signed-off-by: Andy Yan <andy.yan@...k-chips.com>
> > > 
> > > Adding John Stultz to Cc
> > > 
> > > I just saw this thread pop up again, and had to think of John's recent
> > > patch to unify this across platforms.
> > > 
> > > John, can you have a look at this driver too, and see how it fits in?
> > > I think this is yet another variant, using an MMIO register rather than
> > > RAM (as HTC / NVIDIA does) or SRAM (as Qualcomm does), but otherwise
> > > it conceptually fits in with what you had.
> > 
> > FWIW, Tegra typically does use an MMIO register as well. See
> > drivers/soc/tegra/pmc.c:tegra_pmc_restart_notify(). I don't know what
> > HTC does, but if it's writing somewhere in RAM it isn't using the
> > standard way of resetting the SoC. There's early boot ROM code which I
> > think evaluates the PMC_SCRATCH0 register on Tegra to determine which
> > mode to boot into. That's before even any firmware gets the chance of
> > doing anything.
> 
> 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.

Thierry

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