linux-kernel - Re: [PATCH v4] usb: gadget: u_ether: Replace netif_stop_queue with netif_device

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Date: Wed, 17 Apr 2024 17:13:42 +0200
From: Hardik Gajjar <hgajjar@...adit-jv.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
CC: Ferry Toth <fntoth@...il.com>, Hardik Gajjar <hgajjar@...adit-jv.com>,
	<gregkh@...uxfoundation.org>, <s.hauer@...gutronix.de>,
	<jonathanh@...dia.com>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <quic_linyyuan@...cinc.com>,
	<paul@...pouillou.net>, <quic_eserrao@...cinc.com>, <erosca@...adit-jv.com>
Subject: Re: [PATCH v4] usb: gadget: u_ether: Replace netif_stop_queue with
 netif_device_detach

On Tue, Apr 16, 2024 at 04:48:32PM +0300, Andy Shevchenko wrote:
> On Thu, Apr 11, 2024 at 10:52:36PM +0200, Ferry Toth wrote:
> > Op 11-04-2024 om 18:39 schreef Andy Shevchenko:
> > > On Thu, Apr 11, 2024 at 04:26:37PM +0200, Hardik Gajjar wrote:
> > > > On Wed, Apr 10, 2024 at 08:37:42PM +0300, Andy Shevchenko wrote:
> > > > > On Sun, Apr 07, 2024 at 10:51:51PM +0200, Ferry Toth wrote:
> > > > > > Op 05-04-2024 om 13:38 schreef Hardik Gajjar:
> 
> ...
> 
> > > > > > Exactly. And this didn't happen before the 2 patches.
> > > > > > 
> > > > > > To be precise: /sys/class/net/usb0 is not removed and it is a link, the link
> > > > > > target /sys/devices/pci0000:00/0000:00:11.0/dwc3.0.auto/gadget.0/net/usb0 no
> > > > > > longer exists
> > > > So, it means that the /sys/class/net/usb0 is present, but the symlink is
> > > > broken. In that case, the dwc3 driver should recreate the device, and the
> > > > symlink should become active again
> > 
> > Yes, on first enabling gadget (when device mode is activated):
> > 
> > root@...a:~# ls /sys/devices/pci0000:00/0000:00:11.0/dwc3.0.auto/gadget.0/
> > driver  net  power  sound  subsystem  suspended  uevent
> > 
> > Then switching to host mode:
> > 
> > root@...a:~# ls /sys/devices/pci0000:00/0000:00:11.0/dwc3.0.auto/gadget.0/
> > ls: cannot access
> > '/sys/devices/pci0000:00/0000:00:11.0/dwc3.0.auto/gadget.0/': No such file
> > or directory
> > 
> > Then back to device mode:
> > 
> > root@...a:~# ls /sys/devices/pci0000:00/0000:00:11.0/dwc3.0.auto/gadget.0/
> > driver  power  sound  subsystem  suspended  uevent
> > 
> > net is missing. But, network functions:
> > 
> > root@...a:~# ping 10.42.0.1
> > PING 10.42.0.1 (10.42.0.1): 56 data bytes
> > 
> > Mass storage device is created and removed each time as expected.
> 
> So, what's the conclusion? Shall we move towards revert of those two changes?


As promised, I have the tested the this patch with the dwc3 gadget. I could not reproduce
the issue. 

I can see the usb0 exist all the time and accessible regardless of the role switching of the USB mode (peripheral <-> host)

Following are the logs:
//Host to device

console:/sys/bus/platform/devices/a800000.ssusb # echo "peripheral" > mode
console:/sys/bus/platform/devices/a800000.ssusb # ls a800000.dwc3/gadget/net/
usb0

//device to host
console:/sys/bus/platform/devices/a800000.ssusb # echo "host" > mode
console:/sys/bus/platform/devices/a800000.ssusb # ls a800000.dwc3/gadget/net/
usb0
s a800000.dwc3/gadget/net/usb0                                                <
addr_assign_type    duplex             phys_port_name
addr_len            flags              phys_switch_id
address             gro_flush_timeout  power
broadcast           ifalias            proto_down
carrier             ifindex            queues
carrier_changes     iflink             speed
carrier_down_count  link_mode          statistics
carrier_up_count    mtu                subsystem
dev_id              name_assign_type   tx_queue_len
dev_port            netdev_group       type
device              operstate          uevent
dormant             phys_port_id       waiting_for_supplier
console:/sys/bus/platform/devices/a800000.ssusb # ifconfig -a usb0
usb0      Link encap:Ethernet  HWaddr 3a:8b:63:97:1a:9a
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 TX bytes:0

console:/sys/bus/platform/devices/a800000.ssusb #

I strongly advise against reverting the patch solely based on the observed issue of removing the /sys/class/net/usb0 directory while the usb0 interface remains available. 
Instead, I recommend enabling FTRACE to trace the functions involved and identify which faulty call is responsible for removing usb0.

According to current kernel architecture of u_ether driver, only gether_cleanup should remove the usb0 interface along with its kobject and sysfs interface.
I suggest sharing the analysis here to understand why this practice is not followed in your use case or driver ?

I am curious why the driver was developed without adhering to the kernel's gadget architecture.

> 
> > > > I have the dwc3 IP base usb controller, Let me check with this patch and
> > > > share result here.  May be we need some fix in dwc3
> > Would have been nice if someone could test on other controller as well. But
> > another instance of dwc3 is also very welcome.
> > > It's quite possible, please test on your side.
> > > We are happy to test any fixes if you come up with.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>