[PATCH v2 2/3] arm/dt: tegra: add dts file for paz00

[Stephen Warren]

Marc Dietrich wrote at Friday, October 28, 2011 4:30 AM:
> Am Donnerstag, 27. Oktober 2011, 09:50:03 schrieb Stephen Warren:
> > Marc Dietrich wrote at Wednesday, October 26, 2011 1:59 PM:
> > > This adds a dts file for paz00. As a side effect, this also enables
> > > the embedded controller which controls the keyboard, touchpad, power,
> > > leds, and some other functions.
> > ...
> > > +++ b/arch/arm/boot/dts/tegra-paz00.dts
> > > @@ -0,0 +1,70 @@
> > > +/dts-v1/;
> > > +
> > > +/memreserve/ 0x1c000000 0x04000000;
> > > +/include/ "tegra20.dtsi"
> > > +
> > > +/ {
> > > +	model = "Toshiba AC100 / Dynabook AZ";
> > > +	compatible = "compal,paz00", "nvidia,tegra20";
> > > +
> > > +	chosen {
> > > +		bootargs = "mem=448 at 0 console=ttyS0,115200n8 root=/dev/mmcblk1p1";
> >
> > You shouldn't need the mem= parameter here; it wasn't in your first patch set.
> 
> that's because I forgot it. Sorry, I didn't mentioned it in the changelog. I wonder
> why mem= is still needed.

I wonder if this is some conflict between ATAGs and the DT-specified
memory node.

As far as I can tell, the kernel's memory information typically comes
from ATAGs. Some boards override what's in the ATAGs in their machine
descriptor's fixup routine, e.g. see tegra_paz00_fixup(). Presumably,
this is because of buggy bootloaders sending incorrect memory information
in the ATAGs. Do you have any way to check the ATAGs that the bootloader
is sending? Probably, you could enable DEBUG_LL and using the low-level
debugging functions to dump some of that information to the UART early
during boot.

When boards boot from DT, there is no fixup function to override the
bootloader's ATAGs. I also see a bunch of code to set up the memory
information from DT e.g. setup_machine_fdt()'s call to:

	of_scan_flat_dt(early_init_dt_scan_memory, NULL);

... but I assume that happens before the ATAGs are processed, and the
buggy ATAGs end up overriding the information in the DT file. And further,
I assume that specifying "mem=" on the command-line overrides that, thus
solving the problem.

It'd be awesome if you could validate this; the simplest way is probably
to:

a) Remove mem= from the command-line
b) Modify arch/arm/kernel/setup.c:parse_tag_mem32() to do nothing;
   comment out the call to arm_add_memory()
c) Test booting, and check what RAM size the kernel thinks you have.

If that works, then ATAGs are the problem. We probably need to modify the
core ARM code not to use memory ATAGs when there is a DT?

I'm mainly pushing on this because adding "mem=" to the command-line in
the DT file isn't a great solution; when people start using bootloaders
that rewrite the DT to include the user-specified command-line, then every
user is going to have to start specifying "mem=" in their command-lines.

> I've seen patches on the chromeos tree which try to reserve the gpu
> memory on demand. While we are at it, what is the vmalloc=192M used
> by most other boards for?

I'm not sure what vmalloc= does exactly. I somewhat doubt it's necessary
until we have code that uses the GPU in mainline. The same goes for the
/memreserve/ DT entries. I've been thinking of submitting a patch to
remove this cruft, but haven't gotten around to it yet.

-- 
nvpublic