Re: [PATCH v2] usbip: convert to use faux_device

[Zongmin Zhou <min_halo@163.com>]

On 2025/7/11 04:33, Shuah Khan wrote:
> On 7/10/25 08:06, Alan Stern wrote:
>> On Wed, Jul 09, 2025 at 03:57:35PM -0600, Shuah Khan wrote:
>>> On 7/9/25 15:49, Shuah Khan wrote:
>>>> Right. We have a few too many moving pieces here:
>>>>
>>>> usbipd (user-space)
>>>> vhci_hcd and the usb devices it creates
>>>>
>>>> usbip_host, stub driver that proxies between the device
>>>> on the server and vhci_client.
>>>>
>>>> PM can be complex and it has to do lot more than it currently
>>>> does on both server and client end to support seamlessly.
>>>>
>>>> The current suspend took the approach of refusing suspend
>>>> which doesn't work since usb devices underneath hang in
>>>> usb_dev_resume(). Looks like this usb device is treated like
>>>> a real device bu usb core. Is there a way to have usb core
>>>> PM (suspend and resume) handle them as virtual? Would it
>>>> help to use "supports_autosuspend" to disable suspend and
>>>> resume?
>>>
>>> Would it work if usb_disable_autosuspend() on the devices
>>> that hang off its vitual bus?
>>
>> You have to consider PM on both the host and client.  And you have to
>> consider both runtime PM and system PM (that is, suspend to RAM,
>> hibernate, etc.).
>
> This would be as a fix for the existing suspend hang issue.
>
>>
>> On the server, any sort of suspend will interrupt the connection.
>> usb_disable_autosuspend() will prevent runtime suspends, but you
>> shouldn't try to prevent system suspends.  Instead, the usbip driver on
>> the server should have its suspend routine close the connection to the
>> client (rather as if the server's user had unplugged the device).
>>
>> On the client, you've got a choice for how to handle runtime PM.  You
>> can leave it enabled, and when the client decides to suspend the device,
>> tell the server's driver.  The server driver can then attempt to do a
>> runtime suspend on the physical device.  (You might need to add a new
>> type of message to the USBIP protocol to accomplish this; I don't know
>> the details.)  Alternatively, you can forbid runtime suspend on the
>> client entirely, although it would be nice if you could avoid this.
>>
>> System PM on the client can be handled more less the same as runtime PM.
>
> Correct. This has to be a complete solution that syncs up server and 
> client
> side. I am going to look into implementing this - it might be possible to
> do this in user-space. Either way this will require changes to the 
> protocol
> very likely.
>
> Greg, Zongmin Zhou, let's hold off on this conversion yo faux bus for 
> now.
> I will spend time looking at if we can find PM solution that works end 
> to end
> for server and client.
Sorry for the late reply.
Ok,I got it. it is necessary to tackle the
PM standby issue first before considering next steps.
>
> thanks,
> -- Shuah