On Wed, Apr 13, 2016 at 11:38:51PM +0200, Alexander Graf wrote: > On 13.04.16 13:52, Marek Vasut wrote: > > On 04/13/2016 01:22 PM, Beniamino Galvani wrote: > >> On Wed, Apr 13, 2016 at 12:26:43AM +0200, Marek Vasut wrote: > >>>> So, after some investigation, the reason is that the code runs when > >>>> caches are still disabled and thus all the memory is treated as > >>>> Device-nGnRnE, requiring aligned accesses. > >>> > >>> You mean 8-byte aligned accesses, correct ? > >> > >> Yes. > >> > >>>> The return value of > >>>> fdt_getprop() is guaranteed to be aligned to a 4 byte boundary (but > >>>> not 8) > >>> > >>> The return value of fdt_getprop() is a pointer, thus 8byte long on > >>> aarch64 and thus aligned to 8 bytes on the stack unless there is > >>> some real problem. > >> > >> Right, however I'm not talking about the alignment of the pointer on > >> the stack, but about the value of the pointer, which depends on the > >> offset inside the device tree blob of the property. If I use this: > >> > >> val = fdt_getprop(gd->fdt_blob, offset, "reg", &len) > >> gd->ram_size = fdt64_to_cpu(*(fdt64_t *)val) > >> > >> when the CPU tries to dereference val (which is something like > >> 0x00000000010429e4) an alignment fault is generated for the reason > >> stated above. > > > > Oh, now it's clear what the problem is, thanks. But then, we'd need such > > fixups all over the place I'm afraid. Isn't there some way to enable > > support for "unaligned" accesses instead? > > Yes, and it's called "enable the MMU". You could probably do this in the > early dram init stage already, but I'm not sure it's worth it. The NXP > people are the only ones doing it really early today FWIW. > > Also, if you find it more readable, you could just use > get_unaligned_be64(). It gets you down to byte accesses rather than > 32bit fetches, but the function name makes it pretty obvious what we're > looking at.
OK, now I'm starting to get nightmares back to our last unaligned access discussion. Is ARMv8 doing something radically different from ARMv7 here, wrt unaligned accesses? -- Tom
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