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 <tr...@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 <tr...@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
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