Hi Greg,

On Thu, Jul 23, 2020 at 01:07:07PM +0200, Greg Kroah-Hartman wrote:

On Thu, Jul 23, 2020 at 12:27:48PM +0800, Tingwei Zhang wrote:

quoted

When coresight device is in an active session, driver module of
that device should not be removed. Use try_get_module() in
coresight_grab_device() to prevent module to be unloaded.

Are you sure this works?  Why is it needed at all?  Why not just tear
down the children properly when a module is removed so that you don't
need this at all?

Using the terms parent and child is somewhat ambiguous...  This is not a
parent-child relationship but simply an association between devices, something
like port 1 on device "parent" is connected to port 2 on device "child".  The
parent-child nomenclature was chosen to reflect that a device appears before
another in a coresight path.  Otherwise there is no other relation between
devices, hence the choice of using try_get_module()/put_module() to prevent
drivers from being taken away.  I'd be happy to proceed differently but haven't
found better options.

Going back to parent/child, we could have chosen left/right, up/down or A/B, all
of which are just as confusion. 

quoted

Signed-off-by: Tingwei Zhang <redacted>
---
 drivers/hwtracing/coresight/coresight.c | 27 +++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
index b7151c5f81b1..17bc76ea86ae 100644
--- a/drivers/hwtracing/coresight/coresight.c
+++ b/drivers/hwtracing/coresight/coresight.c

@@ -640,7 +640,7 @@ struct coresight_device *coresight_get_sink_by_id(u32 id)
  * don't appear on the trace path, they should be handled along with the
  * the master device.
  */
-static void coresight_grab_device(struct coresight_device *csdev)
+static int coresight_grab_device(struct coresight_device *csdev)
 {
 	int i;
 

@@ -648,10 +648,25 @@ static void coresight_grab_device(struct coresight_device *csdev)
 		struct coresight_device *child;
 
 		child  = csdev->pdata->conns[i].child_dev;
-		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER)
+		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER) {
+			if (!try_module_get(child->dev.parent->driver->owner))

Why the child's parent?  Why not the child itself?

The device structure of each coresight_device is not associated with a driver.
It is there to take advantages of device goodies such as dev.type, dev.group,
dev.release and dev.bus.  Coresight IP blocks are discovered on the AMBA bus and as
such amba_device::dev::driver holds the driver itself.  In coresight_register()
the association coresigth::dev::parent = amba_device::dev is made.

quoted

+				goto err;

What about the error given to you here?  Why throw that away?

quoted

 			pm_runtime_get_sync(child->dev.parent);
+		}
 	}
+	if (!try_module_get(csdev->dev.parent->driver->owner))
+		goto err;

You don't reduce the child's parent's driver owner module reference
here?

Here @parent is referencing the current device.  Now that helper devices
connected to any of its outgoing ports have been enabled (and a reference count 
to the helper device driver incremented), a reference count to the current device
driver can also be incremented. 

I hope this clarify what is going on here.  It is a little unorthodox but so is
coresight.

Thanks,
Mathieu

Ugh, that's a horid sentence :(

quoted

 	pm_runtime_get_sync(csdev->dev.parent);
+	return 0;
+err:
+	for (i--; i >= 0; i--) {
+		struct coresight_device *child;
+
+		child  = csdev->pdata->conns[i].child_dev;
+		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER)
+			pm_runtime_put(child->dev.parent);
+	}
+	return -ENODEV;
 }
 
 /*

@@ -663,12 +678,15 @@ static void coresight_drop_device(struct coresight_device *csdev)
 	int i;
 
 	pm_runtime_put(csdev->dev.parent);
+	module_put(csdev->dev.parent->driver->owner);
 	for (i = 0; i < csdev->pdata->nr_outport; i++) {
 		struct coresight_device *child;
 
 		child  = csdev->pdata->conns[i].child_dev;
-		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER)
+		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER) {
 			pm_runtime_put(child->dev.parent);
+			module_put(child->dev.parent->driver->owner);
+		}
 	}
 }
 

@@ -721,7 +739,8 @@ static int _coresight_build_path(struct coresight_device *csdev,
 	if (!node)
 		return -ENOMEM;
 
-	coresight_grab_device(csdev);
+	if (coresight_grab_device(csdev))
+		return -ENODEV;

Why not return the error given to you?


thanks,

greg k-h

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