From mboxrd@z Thu Jan  1 00:00:00 1970
From: slash.tmp@free.fr (Mason)
Date: Thu, 25 May 2017 00:15:31 +0200
Subject: User-space code aborts on some (but not all) misaligned accesses
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[ Dropping gcc-help at this point ]

On 24/05/2017 19:27, Ard Biesheuvel wrote:

> It is really quite simple
> 1. add the memory to the /memory DT node
> 2. add it as a no-map region to the /reserved-memory DT node
> 
> This should result in pgprot_writecombine() attributes on your O_SYNC
> /dev/mem mapping, which should make the problem go away.

I think I see what you are referring to:

http://elixir.free-electrons.com/linux/latest/source/drivers/char/mem.c#L357

http://elixir.free-electrons.com/linux/latest/source/arch/arm/mm/mmu.c#L701

#ifdef CONFIG_ARM_DMA_MEM_BUFFERABLE
pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
			      unsigned long size, pgprot_t vma_prot)
{
	if (!pfn_valid(pfn))
		return pgprot_noncached(vma_prot);
	else if (file->f_flags & O_SYNC)
		return pgprot_writecombine(vma_prot);
	return vma_prot;
}
EXPORT_SYMBOL(phys_mem_access_prot);
#endif

Telling Linux about the RAM makes pfn_valid() return true,
and using O_SYNC means calling pgprot_writecombine.

Thanks for the pointers.

For my own reference:
https://www.kernel.org/doc/Documentation/unaligned-memory-access.txt
https://www.kernel.org/doc/Documentation/arm/mem_alignment

Regards.