On Thu, Jul 13, 2017 at 12:23:32AM +0300, Pantelis Antoniou wrote:
> This method returns the length of the full path of the node so that
> it may be used without having to call fdt_get_path() using a worst
> case sized buffer.
>
> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>
Hm, I can see why you want this, but the implementation is a bit
horrifyingly slow - even by the standards of the slow functions in
libfdt.
I originally intended for the get_path() function to return the path
length, even if it wouldn't all fit in the buffer. However, I wasn't
able - the buffer into which it builds the path doubles as a sort of
stack keeping track of where we are. It could be replaced by an
explicit stack, which might be smaller than the buffer, but it's still
a data structure of indeterminite size, which is pretty awkward here.
In most cases I'd suggest a user just use get_path(), then retry with
a larger buffer if it fails, but obviously that doesn't really work
from within the overlay application code either. I have some ideas on
how to avoid the need for a get_path_len() which I'll expand on in
that thread.
> ---
> libfdt/fdt_ro.c | 28 ++++++++++++++++++++++++++++
> libfdt/libfdt.h | 22 ++++++++++++++++++++++
> 2 files changed, 50 insertions(+)
>
> diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
> index 08de2cc..8b67f2f 100644
> --- a/libfdt/fdt_ro.c
> +++ b/libfdt/fdt_ro.c
> @@ -438,6 +438,34 @@ int fdt_get_path(const void *fdt, int nodeoffset, char *buf, int buflen)
> return offset; /* error from fdt_next_node() */
> }
>
> +int fdt_get_path_len(const void *fdt, int nodeoffset)
> +{
> + int len = 0, namelen;
> + const char *name;
> +
> + FDT_CHECK_HEADER(fdt);
> +
> + for (;;) {
> + name = fdt_get_name(fdt, nodeoffset, &namelen);
> + if (!name)
> + return namelen;
> +
> + /* root? we're done */
> + if (namelen == 0)
> + break;
> +
> + nodeoffset = fdt_parent_offset(fdt, nodeoffset);
The basic problem is that fdt_parent_offset() is already a slow
function requiring a full scan of the blob, and it's called here
repeatedly.
> + if (nodeoffset < 0)
> + return nodeoffset;
> + len += namelen + 1;
> + }
> +
> + /* in case of root pretend it's "/" */
> + if (len == 0)
> + len++;
> + return len;
> +}
> +
> int fdt_supernode_atdepth_offset(const void *fdt, int nodeoffset,
> int supernodedepth, int *nodedepth)
> {
> diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> index e01c645..2c0b570 100644
> --- a/libfdt/libfdt.h
> +++ b/libfdt/libfdt.h
> @@ -765,6 +765,28 @@ const char *fdt_get_alias(const void *fdt, const char *name);
> int fdt_get_path(const void *fdt, int nodeoffset, char *buf, int buflen);
>
> /**
> + * fdt_get_path_len - determine the length of the full path of a node
> + * @fdt: pointer to the device tree blob
> + * @nodeoffset: offset of the node whose path to find
> + *
> + * fdt_get_path_len() computes the size of the full path of the node at
> + * offset nodeoffset.
> + *
> + * NOTE: This function is expensive, as it must scan the device tree
> + * structure from the start to nodeoffset.
> + *
> + * returns:
> + * > 0, on success
> + * length of the full path of the node
> + * -FDT_ERR_BADOFFSET, nodeoffset does not refer to a BEGIN_NODE tag
> + * -FDT_ERR_BADMAGIC,
> + * -FDT_ERR_BADVERSION,
> + * -FDT_ERR_BADSTATE,
> + * -FDT_ERR_BADSTRUCTURE, standard meanings
> + */
> +int fdt_get_path_len(const void *fdt, int nodeoffset);
> +
> +/**
> * fdt_supernode_atdepth_offset - find a specific ancestor of a node
> * @fdt: pointer to the device tree blob
> * @nodeoffset: offset of the node whose parent to find
--
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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