v4.17 regression: PowerMac G3 won't boot, was Re: [PATCH v5 1/3] of: cache phandle nodes to reduce cost of of_find_node_by_phandle()

[Frank Rowand]

On 09/10/18 05:53, Rob Herring wrote:
> On Sun, Sep 09, 2018 at 07:04:25PM +0200, Benjamin Herrenschmidt wrote:
>> On Fri, 2018-08-31 at 14:58 +1000, Benjamin Herrenschmidt wrote:
>>>
>>>> A long shot, but something to consider, is that I failed to cover the
>>>> cases of dynamic devicetree updates (removing nodes that contain a
>>>> phandle) in ways other than overlays.  Michael Ellerman has reported
>>>> such a problem for powerpc/mobility with of_detach_node().  A patch to
>>>> fix that is one of the tasks I need to complete.
>>>
>>> The only thing I can think of is booting via the BootX bootloader on
>>> those ancient macs results in a DT with no phandles. I didn't see an
>>> obvious reason why that would cause that patch to break though.
>>
>> Guys, we still don't have a fix for this one on its way upstream...
>>
>> My test patch just creates phandle properties for all nodes, that was
>> not intended as a fix, more a way to check if the problem was related
>> to the lack of phandles.
>>
>> I don't actually know why the new code causes things to fail when
>> phandles are absent. This needs to be looked at.
>>
>> I'm travelling at the moment and generally caught up with other things,
>> I haven't had a chance to dig, so just a heads up. I don't intend to
>> submit my patch since it's just a band aid. We need to figure out what
>> the actual problem is.
> 
> Can you try this patch (w/o Ben's patch). I think the problem is if 
> there are no phandles, then roundup_pow_of_two is passed 0 which is 
> documented as undefined result.
> 
> Though, if a DT has no properties with phandles, then why are we doing a 
> lookup in the first place?
> 
> 
> 8<----------------------------------------------------------------------
> 
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index 9095b8290150..74eaedd5b860 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -140,6 +140,9 @@ void of_populate_phandle_cache(void)
>  		if (np->phandle && np->phandle != OF_PHANDLE_ILLEGAL)
>  			phandles++;
>  
> +	if (!phandles)
> +		goto out;
> +
>  	cache_entries = roundup_pow_of_two(phandles);
>  	phandle_cache_mask = cache_entries - 1;
>  
> 

Thanks Rob!  That fix makes sense, and the test results look
promising.

Reviewed-by: Frank Rowand <frank.rowand at sony.com>