linux-kernel - Re: Re: [PATCH] usb: usbfs: Add reset

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Message-ID: <379c80f7-aa01-4575-aa0b-d64ef01c1346@rowland.harvard.edu>
Date: Sun, 24 Aug 2025 22:01:44 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: 自己 <ccc194101@....com>
Cc: gregkh@...uxfoundation.org, jannh@...gle.com, rex.nie@...uarmicro.com,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	chenchangcheng <chenchangcheng@...inos.cn>
Subject: Re: Re: [PATCH] usb: usbfs: Add reset_resume callback to usbfs.

On Mon, Aug 25, 2025 at 09:36:49AM +0800, 自己 wrote:
> At 2025-08-23 04:30:18, "Alan Stern" <stern@...land.harvard.edu> wrote:
> >On Fri, Aug 22, 2025 at 10:46:02AM +0800, ccc194101@....com wrote:
> >> From: chenchangcheng <chenchangcheng@...inos.cn>
> >> 
> >> When an Apple device is inserted into the host, and the host
> >> wakes up from S3/S4 power states, if the reset_resume process
> >> is triggered, the absence of a reset_resume callback in usbfs will
> >> cause the device to unbind.
> >> By adding a reset_resume callback to usbfs and reporting REMOVE and ADD
> >> uevents in reset_resume, the userspace is prompted to reissue commands
> >> to re-establish the binding with usbfs.
> >
> >usbfs has no way to inform userspace when the device is reset.  This is 
> >true for normal resets as well as for reset-resumes (no pre_reset, 
> >post_reset, or reset_resume callbacks).  I don't see any point in trying 
> >to add support for the latter but not the former.
> >
> >Unbinding the device forces userspace to re-open the device file and 
> >establish a new binding.  How does adding REMOVE and ADD uevents make 
> >the situation any better than it already is?
> >
> 
> >Alan Stern
> 
> Here is my reasoning: 
> Currently, for Apple devices after S3/S4 states, since the USB hub loses power, 
> the reset-resume process is triggered during resume. If the original 
> reset_resume process is followed, the device would be forcibly unbound,
> and the device_attach function would be used to rebind the driver.
> However, usbfs is different in that it cannot automatically rebind
> after unbinding and requires a userspace ioctl to re-establish the binding.
> 
> If we assume that the reset_resume callback of usbfs does nothing
> and simply returns 0, the USB device would still be reset. When userspace
> uses the previous file descriptor handle to issue a command, it would result
> in an error: "PTP_OC 0x1007 receiving resp failed: PTP Session Not Open (0x2003)."
> 
> Therefore, by adding REMOVE and ADD uevents in the reset_resume process,
> userspace is notified to first unbind and then rebind. This approach avoids the aforementioned issue.

Doesn't the "PTP Session Not Open" error notify userspace to unbind and 
rebind?  Why is adding REMOVE and ADD uevents any better?

In the current kernel there is no reset_resume callback for usbfs.  
Consequently, when userspace uses the previous file descriptor handle to 
issue an ioctl command after a resume, it gets a -ENODEV error.  Doesn't 
this also notify userspace that it should unbind and rebind?  Why is 
adding a reset_resume callback any better?

Alan Stern