Re: [RFC 1/2] USB: Notify OTG errors to user space via uevents

[Pavan Kondeti <pkondeti@codeaurora.org>]

On 12/7/2010 9:20 AM, Greg KH wrote:
> On Mon, Dec 06, 2010 at 06:32:30PM -0800, pkondeti@codeaurora.org wrote:
>> Hi Greg,
>>
>>> On Mon, Dec 06, 2010 at 06:07:50PM +0530, Pavankumar Kondeti wrote:
>>>> OTG specification mandates no silent failures and all errors should
>>>> be reported to the user.  The spec itself does not give the exact
>>>> error description. But recommends the error message to be self
>>>> explanatory.  Provide otg_notify_error() utility for USB core and
>>>> OTG driver to send the error codes to user space.  All the error
>>>> code values are described in include/linux/usb/ch9.h.  The user space
>>>> application can listen to netlink socket and parse the buffer for
>>>> "MODULE=OTG" and "ERROR=n", where 'n' contains the error code.
>>>
>>> How are you going to listen to the netlink socket that is already
>>> grabbed by libudev?
>>>
>> Sorry. I never worked with udev. But I read udev documentation.
>> I thought an external script can be invoked by adding a udev rule
>> when MODULE=OTG is matched and ERROR value can be accessed
>> in the script via env variable.
> 
> So, you created a new kernel/user ABI and never even tried it out to see
> how someone would even use it?
> 
> {sigh}
> 
> I'm afraid to ask if you actually tested this code, let alone compiled
> the thing...
> 
I have tested this patch with a sample program @
http://www.kernel.org/doc/pending/hotplug.txt on busybox environment
where udevd is not running. After seeing your comments, I ran the same
program concurrently
with udev on Linux box.

>>> Please, if you really want to do this, create your own netlink socket,
>>> don't create new uevent messages that will just confused the existing
>>> tools out there, that's ripe for big problems.
>>>
>> I have seen examples in drivers  sending uevents for notifying docking
>> station status (drivers/acpi/dock.c)
> 
> That is when the hardware configuration of the system changes.
> 
>> and when backlight brightness changes
>> (drivers/video/backlight/backlight.c).
> 
> Again, hardware configuration status changed.
> 
Documentation/filesystems/gfs2-uevents.txt says OFFLINE uevent is sent
upon file system errors.

>> So I have taken this approach.
> 
> Please don't, this is NOT the way to get errors from the kernel back out
> to userspace, because, as you have noted, this is not how any other
> subsystem does it.
> 
Thanks for your feedback. Is there any other way kernel can send errors
to user space asynchronously? is /dev/ interface acceptable?

-- 
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