dm: blk: Add a way to obtain a block device from its parent

Many devices support a child block device (e.g. MMC, USB). Add a
convenient way to get this device given the parent device.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NTom Rini <trini@konsulko.com>

drivers/block/blk-uclass.c

@@ -453,6 +453,32 @@ int blk_prepare_device(struct udevice *dev)
 	return 0;
 }
 
+int blk_get_from_parent(struct udevice *parent, struct udevice **devp)
+{
+	struct udevice *dev;
+	enum uclass_id id;
+	int ret;
+
+	device_find_first_child(parent, &dev);
+	if (!dev) {
+		debug("%s: No block device found for parent '%s'\n", __func__,
+		      parent->name);
+		return -ENODEV;
+	}
+	id = device_get_uclass_id(dev);
+	if (id != UCLASS_BLK) {
+		debug("%s: Incorrect uclass %s for block device '%s'\n",
+		      __func__, uclass_get_name(id), dev->name);
+		return -ENOTBLK;
+	}
+	ret = device_probe(dev);
+	if (ret)
+		return ret;
+	*devp = dev;
+
+	return 0;
+}
+
 int blk_find_max_devnum(enum if_type if_type)
 {
 	struct udevice *dev;

include/blk.h

@@ -616,4 +616,11 @@ ulong blk_write_devnum(enum if_type if_type, int devnum, lbaint_t start,
  */
 int blk_select_hwpart_devnum(enum if_type if_type, int devnum, int hwpart);
 
+/**
+ * blk_get_from_parent() - obtain a block device by looking up its parent
+ *
+ * All devices with
+ */
+int blk_get_from_parent(struct udevice *parent, struct udevice **devp);
+
 #endif

test/dm/blk.c

@@ -150,3 +150,21 @@ static int dm_test_blk_devnum(struct unit_test_state *uts)
 	return 0;
 }
 DM_TEST(dm_test_blk_devnum, DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT);
+
+/* Test that we can get a block from its parent */
+static int dm_test_blk_get_from_parent(struct unit_test_state *uts)
+{
+	struct udevice *dev, *blk;
+
+	ut_assertok(uclass_get_device(UCLASS_MMC, 0, &dev));
+	ut_assertok(blk_get_from_parent(dev, &blk));
+
+	ut_assertok(uclass_get_device(UCLASS_I2C, 0, &dev));
+	ut_asserteq(-ENOTBLK, blk_get_from_parent(dev, &blk));
+
+	ut_assertok(uclass_get_device(UCLASS_GPIO, 0, &dev));
+	ut_asserteq(-ENODEV, blk_get_from_parent(dev, &blk));
+
+	return 0;
+}
+DM_TEST(dm_test_blk_get_from_parent, DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT);