[RFC PATCH] arm64: use fixmap region for permanent FDT mapping

[Mark Rutland]

On Fri, Feb 27, 2015 at 02:53:43PM +0000, Ard Biesheuvel wrote:
> Currently, the FDT blob needs to be in the same naturally aligned
> 512 MB region as the kernel, so that it can be mapped into the
> kernel virtual memory space very early on using a minimal set of
> statically allocated translation tables.
> 
> Now that we have early fixmap support, we can relax this restriction,
> by moving the permanent FDT mapping to the fixmap region instead.
> This way, the FDT blob may be anywhere in memory.

Neat! I was hoping we'd have the chance to do this at some point.

[...]

> +	/*
> +	 * Reserve 2 MB of virtual space for the FDT at the top of the fixmap
> +	 * region. Keep this at the top so it remains 2 MB aligned.
> +	 */
> +#define FIX_FDT_SIZE		SZ_2M
> +	FIX_FDT_END,
> +	FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,

Doesn't the presence of the FIX_HOLE entry mean that this isn't 2MB
aligned?

[...]

>  __create_page_tables:
> -	pgtbl	x25, x26, x28			// idmap_pg_dir and swapper_pg_dir addresses
> +	pgtbl	x25, x26, x28	// idmap_pg_dir and swapper_pg_dir addresses
>  	mov	x27, lr
>  
>  	/*

This comment should just stay where it was. It aligns with comments
later within __create_page_tables that are left alone by this patch, so
we don't gain anything by moving it.

[...]

> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index e8420f635bd4..7ea087d904ad 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -309,12 +309,14 @@ static void __init setup_processor(void)
>  
>  static void __init setup_machine_fdt(phys_addr_t dt_phys)
>  {
> -	if (!dt_phys || !early_init_dt_scan(phys_to_virt(dt_phys))) {
> +	void *dt_virt = fixmap_remap_fdt(dt_phys);
> +
> +	if (!dt_phys || !early_init_dt_scan(dt_virt)) {
>  		early_print("\n"
>  			"Error: invalid device tree blob at physical address 0x%p (virtual address 0x%p)\n"
> -			"The dtb must be 8-byte aligned and passed in the first 512MB of memory\n"
> +			"The dtb must be 8-byte aligned\n"
>  			"\nPlease check your bootloader.\n",
> -			dt_phys, phys_to_virt(dt_phys));
> +			dt_phys, dt_virt);

We should also check that the reported size in the header doesn't cross
a 2MB PA boundary, or we won't be able to map the whole DTB and the
crc32 code will explode later.

Otherwise this looks good!

Thanks,
Mark.