Re: [PATCH] remoteproc: core: Move cdev add before device add

From: Siddharth Gupta
Date: Fri Apr 23 2021 - 17:49:08 EST



On 4/22/2021 8:01 PM, Bjorn Andersson wrote:
On Thu 22 Apr 21:12 CDT 2021, Siddharth Gupta wrote:

On 4/22/2021 11:04 AM, Mathieu Poirier wrote:
Hi Siddharth,

On Tue, Apr 20, 2021 at 12:26:45PM -0700, Siddharth Gupta wrote:
When cdev_add is called after device_add has been called there is no
way for the userspace to know about the addition of a cdev as cdev_add
itself doesn't trigger a uevent notification, or for the kernel to
know about the change to devt. This results in two problems:
- mknod is never called for the cdev and hence no cdev appears on
devtmpfs.
- sysfs links to the new cdev are not established.

Based on how cdev_device_add[1] is written, it appears that the correct
Please don't add this kind of reference to the change log as it will become
invalid with time.
Okay sure, I will remove it.
way to use these APIs is to call cdev_add before device_add is called.
Since the cdev is an optional feature for remoteproc we cannot directly
use the existing API.
Please explain why the existing API can't be used directly.
Not sure what you mean here Mathieu? The reason why we can't use
it is because cdev is an optional feature. We would either have to move
device_add inside rproc_char_dev_add or the other way around and
make cdev a regular feature. Since device_add can't be called on the
same device struct twice[1], we have to do things this way. Also this
way we don't have to rely on the userspace to call mknod as it will
be called[2] as a part of the device_add call in devtmpfs_create_node.

Now that I think about it, is the above what you want me to put in the
commit text? :)

Your patch is correct, we need to cdev_add() and in particular assign
dev->devt before calling device_add(). Given how the code is split in
core and cdev there's no way to use cdev_device_add(), but I don't think
anyone is suggesting that - except for your commit message.

So while everything you mention in your commit message seems correct,
you should be able to make it more to the point by distilling it down to
something like:

The cdev needs to be added and devt assigned before device_add() is
called in order for the relevant sysfs and devtmpfs entries to be
created and the uevent to be properly populated.
Okay, I will make the appropriate changes. Thanks!
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/base/core.c#n3105
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/base/devtmpfs.c#n215
Hence moving rproc_char_device_add() before
device_add() in rproc_add().

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/char_dev.c#n537

Signed-off-by: Siddharth Gupta <sidgup@xxxxxxxxxxxxxx>
---
drivers/remoteproc/remoteproc_core.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
index 626a6b90f..562355a 100644
--- a/drivers/remoteproc/remoteproc_core.c
+++ b/drivers/remoteproc/remoteproc_core.c
@@ -2316,6 +2316,11 @@ int rproc_add(struct rproc *rproc)
struct device *dev = &rproc->dev;
int ret;
+ /* add char device for this remoteproc */
+ ret = rproc_char_device_add(rproc);
+ if (ret < 0)
+ return ret;
+
I have tested this change and it works. So how did it work before?
It is a sporadic issue due to a race between the userspace uevent handler
and cdev_add. If the uevent for the device is received/processed after
cdev_add the cdev is created.

If "add" is written to the uevent file or mknod is manually called for devt
the cdev works as expected, just that the sysfs links won't be created.
So it works for e.g. systemd based systems (most of the time), while in
a system based on devtmpfs the dev node would never show up.
Yes exactly! We were just lucky till now :)

Thanks,
Sid
Regards,
Bjorn

ret = device_add(dev);
if (ret < 0)
return ret;
@@ -2329,11 +2334,6 @@ int rproc_add(struct rproc *rproc)
/* create debugfs entries */
rproc_create_debug_dir(rproc);
- /* add char device for this remoteproc */
- ret = rproc_char_device_add(rproc);
- if (ret < 0)
- return ret;
-
While reviewing this patch I had another look at rproc_add() and noticed it
doesn't clean up after itself in case of failure. If any of the conditions
aren't met the function returns but rproc_delete_debug_dir(),
rproc_char_device_remove() and device_del() aren't called. Please fix that as
part of your next revision.
Sure. I'll make those changes.

Thanks,
Sid
Thanks,
Mathieu


/* if rproc is marked always-on, request it to boot */
if (rproc->auto_boot) {
ret = rproc_trigger_auto_boot(rproc);
--
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