[PATCH for-7.2 10/10] hmp, device_tree.c: add 'info fdt <property>' support

[Daniel Henrique Barboza]

'info fdt' is only able to print full nodes so far. It would be good to
be able to also print single properties, since ometimes we just want
to verify a single value from the FDT.

libfdt does not have support to find a property given its full path, but
it does have a way to return a fdt_property given a prop name and its
subnode.

This is how we're going to support it:

- given the same fullpath parameter, assume it's a node. If we have a
match with an existing node, print it. If not, assume it's a property;

- in fdt_find_property() we're going to split 'fullpath' into node and
property. Unfortunately we can't use g_path_get_basename() to helps us
because, although the device tree path format is similar to Linux, it'll
not work when trying to run QEMU under Windows where the path format is
different;

- after spliiting into node + property, try to find the node in the FDT.
If we have a match, use fdt_get_property() to retrieve fdt_property.
Return it if found;

- using the fdt_print_property() created previously, print the property.

After this change, if an user wants to print just the value of 'cpu' inside
/cpu/cpu-map(...) from an ARM FDT, we can do it:

(qemu) info fdt /cpus/cpu-map/socket0/cluster0/core0/cpu
/cpus/cpu-map/socket0/cluster0/core0/cpu = <0x8001>
(qemu)

Or the 'ibm,my-dma-window' from the v-scsi device inside the pSeries
FDT:

(qemu) info fdt /vdevice/v-scsi@71000003/ibm,my-dma-window
/vdevice/v-scsi@71000003/ibm,my-dma-window = <0x71000003 0x0 0x0 0x0 0x10000000>
(qemu)

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 hmp-commands-info.hx  |  2 +-
 softmmu/device_tree.c | 79 ++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 76 insertions(+), 5 deletions(-)

diff --git a/hmp-commands-info.hx b/hmp-commands-info.hx
index abf277be7d..8891c2918a 100644
--- a/hmp-commands-info.hx
+++ b/hmp-commands-info.hx
@@ -913,7 +913,7 @@ ERST
         .name       = "fdt",
         .args_type  = "fullpath:s",
         .params     = "fullpath",
-        .help       = "show firmware device tree node given its full path",
+        .help       = "show firmware device tree node or property given its 
full path",
         .cmd        = hmp_info_fdt,
     },
 
diff --git a/softmmu/device_tree.c b/softmmu/device_tree.c
index e41894fbef..f6eb060acc 100644
--- a/softmmu/device_tree.c
+++ b/softmmu/device_tree.c
@@ -774,9 +774,74 @@ static void fdt_print_node(int node, int depth, const char 
*fullpath)
     qemu_printf("%*s}\n", padding, "");
 }
 
+static const struct fdt_property *fdt_find_property(const char *fullpath,
+                                                    int *prop_size,
+                                                    Error **errp)
+{
+    const struct fdt_property *prop = NULL;
+    void *fdt = current_machine->fdt;
+    g_autoptr(GString) nodename = NULL;
+    const char *propname = NULL;
+    int path_len = strlen(fullpath);
+    int node = 0; /* default to root node '/' */
+    int i, idx = -1;
+
+    /*
+     * We'll assume that we're dealing with a property. libfdt
+     * does not have an API to find a property given the full
+     * path, but it does have an API to find a property inside
+     * a node.
+     */
+    nodename = g_string_new("");
+
+    for (i = path_len - 1; i >= 0; i--) {
+        if (fullpath[i] == '/') {
+            idx = i;
+            break;
+        }
+    }
+
+    if (idx == -1) {
+        error_setg(errp, "FDT paths must contain at least one '/' character");
+        return NULL;
+    }
+
+    if (idx == path_len - 1) {
+        error_setg(errp, "FDT paths can't end with a '/' character");
+        return NULL;
+    }
+
+    propname = &fullpath[idx + 1];
+
+    if (idx != 0) {
+        g_string_append_len(nodename, fullpath, idx);
+
+        node = fdt_path_offset(fdt, nodename->str);
+        if (node < 0) {
+            error_setg(errp, "node '%s' of property '%s' not found in FDT",
+                       nodename->str, propname);
+            return NULL;
+        }
+    } else {
+        /* idx = 0 means that it's a property of the root node */
+        g_string_append(nodename, "/");
+    }
+
+    prop = fdt_get_property(fdt, node, propname, prop_size);
+    if (!prop) {
+        error_setg(errp, "property '%s' not found in node '%s' in FDT",
+                   propname, nodename->str);
+        return NULL;
+    }
+
+    return prop;
+}
+
 void fdt_info(const char *fullpath, Error **errp)
 {
-    int node;
+    const struct fdt_property *prop = NULL;
+    Error *local_err = NULL;
+    int node, prop_size;
 
     if (!current_machine->fdt) {
         error_setg(errp, "Unable to find the machine FDT");
@@ -784,10 +849,16 @@ void fdt_info(const char *fullpath, Error **errp)
     }
 
     node = fdt_path_offset(current_machine->fdt, fullpath);
-    if (node < 0) {
-        error_setg(errp, "node '%s' not found in FDT", fullpath);
+    if (node >= 0) {
+        fdt_print_node(node, 0, fullpath);
+        return;
+    }
+
+    prop = fdt_find_property(fullpath, &prop_size, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     }
 
-    fdt_print_node(node, 0, fullpath);
+    fdt_print_property(fullpath, prop->data, prop_size, 0);
 }
-- 
2.36.1