I can see that arm64 requires 8 bytes. That is stated in section 2 of https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm64/booting.rst.
I can't see a similar requirement for arm, although my search was not exhaustive. More generally I can see that all device trees must be at least 4 byte aligned (from section II.2 of https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/booting-without-of.rst) So it does seem that 8 bytes would work for at least both of these. I would be happy with hard-coding that, as I doubt it would cause any problems with other architectures. I don't have anything to add on the ability to relocate the device tree. In my case the device tree is for the next stage u-boot, so won't need relocating. This might become an issue if this was booting direct to linux from the SPL perhaps. > -----Original Message----- > From: Tom Rini <tr...@konsulko.com> > Sent: Tuesday, 25 August 2020 8:40 am > To: Reuben Dowle <reuben.do...@4rf.com> > Cc: u-boot@lists.denx.de > Subject: Re: [PATCH] Fix data abort caused by mis-aligning fit data in > > On Mon, Aug 24, 2020 at 08:05:24PM +0000, Reuben Dowle wrote: > > > Should I submit a new patch with the alignment set to 8 bytes? I would > think a hard coded 8 bytes would not be the best solution, since not all > architectures will need that much alignment. I suspect some would work with > any alignment, and most 32-bit archs would be fine with 4-byte alignment. > > > > Our released software is actually using a patch to align to 4096 bytes, but > > I > knew that was unnecessarily large. I was not really sure what would be an > appropriate value here, and took a guess at ARCH_DMA_MINALIGN when I > cleaned it up for submitting upstream. Is there a better define to use? > > > > I am also interested to know where the 8 byte alignment requirement is > documented. > > So we're talking about the device tree file, and only that, in this part > of the code, right? In the Linux kernel documentation, both arm and > arm64 document that the device tree must be on an 8-byte aligned > address. That is the bare minimum. If we aren't further relocating it > (as fdt_high is set to 0xffffffff for example, which in general is wrong > and bad), that's still the best we can do. It would be good to allow > for further relocation down the line as we aren't making sure it > wouldn't be overwritten by the kernel BSS, etc. > > -- > Tom
