[PATCH kernel] RFC: prom_init: Fetch flatten device tree from the system firmware

[Michael Ellerman]

Alexey Kardashevskiy <aik at ozlabs.ru> writes:

> At the moment, on 256CPU + 256 PCI devices guest, it takes the guest
> about 8.5sec to read the entire device tree. Some explanation can be
> found here: https://patchwork.ozlabs.org/patch/826124/ but mostly it is
> because the kernel traverses the tree twice and it calls "getprop" for
> each properly which is really SLOF as it searches from the linked list
> beginning every time.
>
> Since SLOF has just learned to build FDT and this takes less than 0.5sec
> for such a big guest, this makes use of the proposed client interface
> method - "fdt-fetch".

It's a pity doing it the normal way is so slow, but this seems like a
reasonable idea anyway.

> diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
> index 02190e90c7ae..daa50a153737 100644
> --- a/arch/powerpc/kernel/prom_init.c
> +++ b/arch/powerpc/kernel/prom_init.c
> @@ -2498,6 +2498,31 @@ static void __init flatten_device_tree(void)
>  		prom_panic("Can't allocate initial device-tree chunk\n");
>  	mem_end = mem_start + room;
  
I'd prefer you didn't munge it inside flatten_device_tree(), rather
create a wrapper that does ~=:

void get_flat_devicetree(void)
{
	if (!fetch_flat_devicetree())
        	flatten_device_tree();

	printf(...)
}

> +	if (!call_prom_ret("fdt-fetch", 2, 1, NULL, mem_start,
> +			   room - sizeof(mem_reserve_map))) {
> +		u32 size;
> +
> +		hdr = (void *) mem_start;
> +
> +		/* Fixup the boot cpuid */
> +		hdr->boot_cpuid_phys = cpu_to_be32(prom.cpu);
> +
> +		/* Append the reserved map to the end of the blob */
> +		hdr->off_mem_rsvmap = hdr->totalsize;
> +		size = be32_to_cpu(hdr->totalsize);
> +		rsvmap = (void *) hdr + size;
> +		hdr->totalsize = cpu_to_be32(size + sizeof(mem_reserve_map));
> +		memcpy(rsvmap, mem_reserve_map, sizeof(mem_reserve_map));
> +
> +		/* Store the DT address */
> +		dt_header_start = mem_start;
> +
> +#ifdef DEBUG_PROM
> +		prom_printf("Fetched DTB: %d bytes to @%x\n", size, mem_start);
> +#endif

I think that should actually not be under DEBUG_PROM. The origin of the
FDT is fairly crucial information, so I think we can tolerate an extra
line of output to know that.

> +		goto print_exit;

This was the clue that it should be in a separate function :)

cheers

> +	}
> +
>  	/* Get root of tree */
>  	root = call_prom("peer", 1, 1, (phandle)0);
>  	if (root == (phandle)0)
> @@ -2548,6 +2573,7 @@ static void __init flatten_device_tree(void)
>  	/* Copy the reserve map in */
>  	memcpy(rsvmap, mem_reserve_map, sizeof(mem_reserve_map));
>  
> +print_exit:
>  #ifdef DEBUG_PROM
>  	{
>  		int i;
> -- 
> 2.11.0