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

[Alexey Kardashevskiy]

On 16/10/17 17:11, David Gibson wrote:
> 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.


Ah, I did not realize it is just a phandle from /chosen/cpu. Will fix.


> 
>> +		/* 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. 

? Get the blob, increase the FDT size by sizeof(mem_reserve_map), fix up
totalsize and off_mem_rsvmap, copy mem_reserve_map to the end of the blob
(the actual order is slightly different, may be a bit confusing).

Asking SLOF to reserve the space seems to be unnecessary complication of
the interface - SLOF does not provide any reserved memory records.

> Note also that the reserve map is required to
> be 8-byte aligned, which totalsize might not be.

Ah, good point.


> 
>> +		/* 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;
> 


-- 
Alexey

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