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

[David Gibson]

On Mon, Oct 16, 2017 at 04:49:17PM +1100, Alexey Kardashevskiy wrote:
> 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".
> 
> If "fdt-fetch" is not available, the old method is used.
> 
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>

I like the concept, few details though..

> ---
>  arch/powerpc/kernel/prom_init.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> 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;
>  
> +	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);

If SLOF is generating a tree it really should get this header field
right as well.

> +		/* 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));

.. and the reserve map for that matter.  I don't really understand
what you're doing here.  Note also that the reserve map is required to
be 8-byte aligned, which totalsize might not be.

> +		/* 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
> +		goto print_exit;
> +	}
> +
>  	/* 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;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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