Re: [RFC PATCH v2 1/6] device_tree: Add qemu_fdt_add_path

[David Gibson]

On Sat, Apr 17, 2021 at 10:36:20AM +0800, wangyanan (Y) wrote:
> Hi David,
> 
> On 2021/4/16 12:52, David Gibson wrote:
> > On Tue, Apr 13, 2021 at 04:07:40PM +0800, Yanan Wang wrote:
> > > From: Andrew Jones <drjones@redhat.com>
> > > 
> > > qemu_fdt_add_path() works like qemu_fdt_add_subnode(), except
> > > it also adds any missing subnodes in the path. We also tweak
> > > an error message of qemu_fdt_add_subnode().
> > > 
> > > We'll make use of this new function in a coming patch.
> > > 
> > > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > > Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> > > ---
> > >   include/sysemu/device_tree.h |  1 +
> > >   softmmu/device_tree.c        | 45 ++++++++++++++++++++++++++++++++++--
> > >   2 files changed, 44 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/sysemu/device_tree.h b/include/sysemu/device_tree.h
> > > index 8a2fe55622..ef060a9759 100644
> > > --- a/include/sysemu/device_tree.h
> > > +++ b/include/sysemu/device_tree.h
> > > @@ -121,6 +121,7 @@ uint32_t qemu_fdt_get_phandle(void *fdt, const char 
> > > *path);
> > >   uint32_t qemu_fdt_alloc_phandle(void *fdt);
> > >   int qemu_fdt_nop_node(void *fdt, const char *node_path);
> > >   int qemu_fdt_add_subnode(void *fdt, const char *name);
> > > +int qemu_fdt_add_path(void *fdt, const char *path);
> > >   #define qemu_fdt_setprop_cells(fdt, node_path, property, ...)           
> > >       \
> > >       do {                                                                
> > >       \
> > > diff --git a/softmmu/device_tree.c b/softmmu/device_tree.c
> > > index 2691c58cf6..8592c7aa1b 100644
> > > --- a/softmmu/device_tree.c
> > > +++ b/softmmu/device_tree.c
> > > @@ -541,8 +541,8 @@ int qemu_fdt_add_subnode(void *fdt, const char *name)
> > >       retval = fdt_add_subnode(fdt, parent, basename);
> > >       if (retval < 0) {
> > > -        error_report("FDT: Failed to create subnode %s: %s", name,
> > > -                     fdt_strerror(retval));
> > > +        error_report("%s: Failed to create subnode %s: %s",
> > > +                     __func__, name, fdt_strerror(retval));
> > >           exit(1);
> > >       }
> > > @@ -550,6 +550,47 @@ int qemu_fdt_add_subnode(void *fdt, const char *name)
> > >       return retval;
> > >   }
> > > +/*
> > > + * Like qemu_fdt_add_subnode(), but will add all missing
> > > + * subnodes in the path.
> > > + */
> > > +int qemu_fdt_add_path(void *fdt, const char *path)
> > > +{
> > > +    char *dupname, *basename, *p;
> > > +    int parent, retval = -1;
> > > +
> > > +    if (path[0] != '/') {
> > > +        return retval;
> > > +    }
> > > +
> > > +    parent = fdt_path_offset(fdt, "/");
> > Getting the offset for "/" is never needed - it's always 0.
> Thanks, will fix it.
> > > +    p = dupname = g_strdup(path);
> > You shouldn't need the strdup(), see below.
> > 
> > > +
> > > +    while (p) {
> > > +        *p = '/';
> > > +        basename = p + 1;
> > > +        p = strchr(p + 1, '/');
> > > +        if (p) {
> > > +            *p = '\0';
> > > +        }
> > > +        retval = fdt_path_offset(fdt, dupname);
> > The fdt_path_offset_namelen() function exists *exactly* so that you
> > can look up partial parths without having to mangle your input
> > string.  Just set the namelen right, and it will ignore anything to
> > the right of that.
> Function fdt_path_offset_namelen() seems more reasonable.
> 
> After we call qemu_fdt_add_path() to add "/cpus/cpu-map/socket0/core0"
> successfully,
> if we want to add another path like "/cpus/cpu-map/socket0/core1" we will
> get the error
> -FDT_ERR_NOTFOUND for each partial path. But actually
> "/cpus/cpu-map/socket0"
> already exists, so by using fdt_path_offset_namelen() with right namelen we
> can avoid
> the error retval for this part.

I don't quite follow what you're saying here.  AFAICT your logic was
correct - it just involved a lot of mangling the given path (adding
and removing \0s) which becomes unnecessary with
fdt_path_offset_namelen().

> > > +        if (retval < 0 && retval != -FDT_ERR_NOTFOUND) {
> > > +            error_report("%s: Invalid path %s: %s",
> > > +                         __func__, path, fdt_strerror(retval));
> > If you're getting an error other than FDT_ERR_NOTFOUND here, chances
> > are it's not an invalid path, but a corrupted fdt blob or something
> > else.
> 
> Right, there can be variable reasons for the fail in addition to the invalid
> path.
> 
> > > +            exit(1);
> > > +        } else if (retval == -FDT_ERR_NOTFOUND) {
> > > +            retval = fdt_add_subnode(fdt, parent, basename);
> > > +            if (retval < 0) {
> > > +                break;
> > > +            }
> I found another question here. If path "/cpus/cpu-map/socket0/core0" has
> already
> been added, when we want to add another path "/cpus/cpu-map/socket0/core1"
> and go here with retval = fdt_add_subnode(fdt, parent, "cpus"), then retval
> will
> be -FDT_ERR_EXISTS, but we can't just break the loop in this case.
> 
> Am I right of the explanation ?

Not exactly.  If you called that fdt_add_subnode() in that case you
would get FDT_ERR_EXISTS.  However, because the previous
fdt_path_offset() has returned -FDT_ERR_NOTFOUND, you've already
established that the partial path doesn't exist, so if you got an
FDT_ERR_EXISTS here something has gone very strangely wrong (such as
concurrent access to the flat tree, which would very likely corrupt
it).

Note that the repeated use of fdt_path_offset() in this function will
repeatedly rescan the whole fdt from the beginning.  If you're
concerned about performance (which you might well not be), then a more
efficient approach would be to take your input path component by
component and use fdt_subnode_offset() directly.

-- 
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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