[PATCH 0/9] Switch internal registers address to 0xF1 on Armada 370/XP

[thomas.petazzoni@free-electrons.com (Thomas Petazzoni)]

Dear Andrew Lunn,

On Wed, 22 May 2013 17:51:34 +0200, Andrew Lunn wrote:

> Maybe i'm missunderstand Arnd, but i think he is suggesting a property
> somewhere in DT which says where the bootloader left the register
> space before jumping to the kernel. You can then look at this property
> and then decide if you need to remap or not. Only this one property
> needs to differ between old and new bootloader. You could even say, if
> the property is not in DT, a remap is needed.

The problem is that neither the old nor the new bootloaders are setting
such property in the DT. Old bootloaders shipped by Marvell don't have
DT support at all. New bootloaders shipped by Marvell do have DT
support, but they are not adding any specific DT property that allows
us to tell whether the registers are mapped here or there.

We could maybe find a way of detecting whether if the bootloader is DT
capable, and assume that if it's DT capable, the registers are already
mapped at 0xf1. But that's very fragile: nothing prevents an user to
use a DT-capable bootloader to boot a kernel in the old style way
(appended DTB). In this case, we would assume that the registers are at
0xd0, because we're being booted old-style, and therefore it must be an
old bootloader. Very fragile, clearly not better than the CP15 trick.

> However, i don't know if this can actually work. How early can you
> parse the DT in order to know where the serial port is for
> earlyprintk()? It also gets messy keeping track of the two different
> DT binary blobs and somehow having the probe the uboot version in
> order to install the right one. I don't know how i would include this
> into the debian flash-kernel for example.

For earlyprintk, we can "solve" it as Arnd suggested: just have two
Kconfig options, one for 0xd0, one for 0xf1, and it's up to the user to
choose the right one. It's not entirely nice because it's no longer
automatic, and I'm sure our users will have a hard time figuring out
whether they have an "old" or a "new" bootloader. But let's assume we
do this. earlyprintk knows the physical address of the UART.

Then, we enter ->map_io(). We have access to the Device Tree. But the
Device Tree doesn't tell us whether the bootloader has mapped the
registers. As I explained to Arnd, it's not a per-board criteria: when
I'm booting my OpenBlocks with the manufacturer U-Boot, registers are
mapped at 0xd0. When I boot the same OpenBlocks with the (admittedly
work-in-progress) Barebox bootloader, registers are mapped at 0xf1.
Same board, different register location.

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com