[PATCH 1/2] dm: core: add support for fallback drivers

[Caleb Connolly]

Introduce support for a uclass to provide a fallback/stub driver which
can be used when no device is found for a given node. This might be
useful for handling non-essential clock controllers like the RPMh on
Qualcomm platforms, or during early bringup to get UART output before a
real clock driver has been created.

Signed-off-by: Caleb Connolly <caleb.connolly at linaro.org>
---
 drivers/core/Kconfig  | 10 ++++++++++
 drivers/core/uclass.c | 24 +++++++++++++++++++++++-
 include/dm/uclass.h   |  3 +++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index 1081d61fcf01..09075b9b7a15 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -466,5 +466,15 @@ config BOUNCE_BUFFER
 
 	  A second possible use of bounce buffers is their ability to
 	  provide aligned buffers for DMA operations.
 
+menuconfig FALLBACK_DRIVERS
+	bool "Enable per-uclass fallback drivers"
+	depends on DM
+	help
+	  If a driver requests a resource (like a clock) from a node which
+	  isn't bound to a driver, the driver model will look for a fallback
+	  driver to "stub" the resource. These stubs usually do nothing and
+	  are therefore only suitable in instances where the resource is not
+	  required.
+
 endmenu
diff --git a/drivers/core/uclass.c b/drivers/core/uclass.c
index e46d5717aa62..91d3a48d77b8 100644
--- a/drivers/core/uclass.c
+++ b/drivers/core/uclass.c
@@ -378,8 +378,26 @@ int uclass_find_device_by_of_offset(enum uclass_id id, int node,
 
 	return -ENODEV;
 }
 
+static int uclass_bind_fallback(struct uclass *uc, ofnode node, struct udevice **devp)
+{
+	struct driver *drv;
+
+	log(LOGC_DM, LOGL_ERR, "   - binding fallback '%s' driver '%s'\n",
+	    uc->uc_drv->name, uc->uc_drv->fallback_drv_name);
+
+	drv = lists_driver_lookup_name(uc->uc_drv->fallback_drv_name);
+	if (!drv) {
+		log(LOGC_DM, LOGL_DEBUG, "   - Can't find stub driver '%s' for uclass '%s'\n",
+		    uc->uc_drv->fallback_drv_name, uc->uc_drv->name);
+		return -ENOENT;
+	}
+
+	return device_bind_with_driver_data(gd->dm_root, drv,
+					  ofnode_get_name(node), 0, node, devp);
+}
+
 int uclass_find_device_by_ofnode(enum uclass_id id, ofnode node,
 				 struct udevice **devp)
 {
 	struct uclass *uc;
@@ -401,9 +419,13 @@ int uclass_find_device_by_ofnode(enum uclass_id id, ofnode node,
 			*devp = dev;
 			goto done;
 		}
 	}
-	ret = -ENODEV;
+
+	if (CONFIG_IS_ENABLED(FALLBACK_DRIVERS) && uc->uc_drv->fallback_drv_name)
+		ret = uclass_bind_fallback(uc, node, devp);
+	else
+		ret = -ENODEV;
 
 done:
 	log(LOGC_DM, LOGL_DEBUG, "   - result for %s: %s (ret=%d)\n",
 	    ofnode_get_name(node), *devp ? (*devp)->name : "(none)", ret);
diff --git a/include/dm/uclass.h b/include/dm/uclass.h
index 456eef7f2f31..b99e36485bc5 100644
--- a/include/dm/uclass.h
+++ b/include/dm/uclass.h
@@ -67,8 +67,10 @@ struct udevice;
  * @child_pre_probe: Called before a child in this uclass is probed
  * @child_post_probe: Called after a child in this uclass is probed
  * @init: Called to set up the uclass
  * @destroy: Called to destroy the uclass
+ * @stub_drv_name: Name of a stub driver to use for devices that are not
+ * supported by any other driver.
  * @priv_auto: If non-zero this is the size of the private data
  * to be allocated in the uclass's ->priv pointer. If zero, then the uclass
  * driver is responsible for allocating any data required.
  * @per_device_auto: Each device can hold private data owned
@@ -98,8 +100,9 @@ struct uclass_driver {
 	int (*child_pre_probe)(struct udevice *dev);
 	int (*child_post_probe)(struct udevice *dev);
 	int (*init)(struct uclass *class);
 	int (*destroy)(struct uclass *class);
+	const char *fallback_drv_name;
 	int priv_auto;
 	int per_device_auto;
 	int per_device_plat_auto;
 	int per_child_auto;

-- 
2.44.0