[U-Boot] [RFC] ARM: Start using fdt_high for relocation

[Tom Rini]

On Fri, Dec 06, 2013 at 08:28:04PM +0100, Albert ARIBAUD wrote:
> Hi Tom,
> 
> On Fri, 6 Dec 2013 12:18:13 -0500, Tom Rini <trini at ti.com> wrote:
> 
> > On Fri, Dec 06, 2013 at 05:59:37PM +0100, Albert ARIBAUD wrote:
> > > Hi Tom,
> > > 
> > > On Fri, 6 Dec 2013 10:48:50 -0500, Tom Rini <trini at ti.com> wrote:
> > > 
> > > > Hey all,
> > > > 
> > > > I've been thinking.  We've had a thread on i.MX platforms about fdt
> > > > being overwritten and needing to be moved to another address.  And I've
> > > > also had an internal problem about fdt being overwritten.  So, how about
> > > > as a rule of thumb we start setting fdt_high (in configs) to
> > > > memory-start + 512MiB, as that's the lowmem limit we should always have
> > > > available.  This will fix the problem of BSS overwriting the DT, which
> > > > is the problem we won't catch in normal bootm/bootz usage.  Thoughts?
> > > 
> > > Not sure I'm getting the issue clear, and I would like to avoid (me and
> > > others) having to switch back and forth between threads. Can you sketch
> > > the failure scenario in a couple of lines?
> > 
> > Sure.  Lets take am335x_evm builds (so Beaglebone Black/White, etc).
> > If you start enabling all of the tracing options in the kernel (function
> > tracing, graphs, etc), you get an uncompressed kernel and BSS that will
> > use up the first ~16MiB of DDR.  We default to placing the DT at about
> > 15MiB into memory.  So the kernel runs, clears BSS and eats the DT.
> > System now hangs, and depending on debug options set you may or may not
> > see anything at all from the kernel.  U-Boot couldn't detect this
> > failure because we don't know how big the kernel BSS is, only how big
> > the zImage is (and where it is) and how big the fdt is and where it is.
> > No overlaps, go ahead and run.
> 
> Thanks.
> 
> The only issue I have with the RFC is that the +512 MiB value will only
> work with targets which have more than 512 MiB DDR, right? But since
> you're suggesting this should be set in configs, you are only suggesting
> +512 MiB, and any target could actually specify a lower value as long as
> it's greater than or eqal to 16 MiB. Correct?

fdt_high is only an upper bound on what we may relocate to (setting
aside the magic value of 0xffffffff which means no relocation).  I just
confirmed this too:
U-Boot# bdi
...
boot_params = 0x80000100
DRAM bank   = 0x00000000
-> start    = 0x80000000
-> size     = 0x40000000
...
U-Boot# setenv fdt_high 0xe0000000
...
U-Boot# bootz $loadaddr - $fdtaddr
Kernel image @ 0x80200000 [ 0x000000 - 0x3e5068 ]
## Flattened Device Tree blob at 80f80000
   Booting using the fdt blob at 0x80f80000
   Loading Device Tree to bf62a000, end bf630d9a ... OK

Of course, 0xbf62a000 is a bad address in that it won't be seen by the
kernel, but it was in the higher-than-we-have-memory-at limit I set.

-- 
Tom
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 836 bytes
Desc: Digital signature
URL: <http://lists.denx.de/pipermail/u-boot/attachments/20131206/947a51e0/attachment.pgp>