Re: [Xen-devel] [PATCH v3 15/24] xen/dts: Provide an helper to get a DT node from a path provided by a guest

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-03-10 at 13:58 +0000, Julien Grall wrote:
> On 23/02/15 16:27, Ian Campbell wrote:
> > On Mon, 2015-02-23 at 16:01 +0000, Julien Grall wrote:
> >> Hi Ian,
> >>
> >> On 23/02/15 15:30, Ian Campbell wrote:
> >>> On Tue, 2015-01-13 at 14:25 +0000, Julien Grall wrote:
> >>>
> >>>> +/* This limit is used by the hypercalls to restrict the size of the 
> >>>> path */
> >>>> +#define DEVICE_TREE_MAX_PATHLEN 1024
> >>>
> >>> Is this something you've made up or derived from the DT spec/ePAPR etc?
> >>
> >> I didn't find a such requirements on the spec.
> > 
> > I vaguely recall a limit on the length of a single node name, and a
> > limit on the depth which they may nest, which can be multiplied. It's
> > probably an unhelpfully large number though, so...
> > 
> >> I chose this number based on the linux pathlen because the path is also
> >> used in the sysfs (/sys/firmware/devicetree).
> > 
> > ...that's a good as anything I suppose!
> 
> Hmmm... I'm not so sure about the 1024 limit anymore. Linux is defining
> PATH_MAX to 4096 but I don't see many usage in the sysfs code.
> 
> And this value may change from one OS to another. Although, 1024 sounds
> a very long path to write in the configuration file...

I imagine in practice it will be cut-and-pasted, but yes.

>  Maybe we should
> support alias too?

Perhaps 2048 + (optionally, perhaps later) add alias support?

2048 because I assume 4096 being PAGE_SIZE would have some interesting
corner cases, especially for a target buffer on the stack. If not then
why not go straight for 4096?

Ian.


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