[PATCH] usbnet: Set duplex status to unknown in the absence of MII

[PATCH] usbnet: Set duplex status to unknown in the absence of MII

[yicongsrfy]

From: Yi Cong <yicong@kylinos.cn>

CDC device don't use mdio to get link status, duplex is set
half as default.

Now cdc_ncm can't get duplex, set it UNKNOWN instead of half
which might actually be in an error state.

Signed-off-by: Yi Cong <yicong@kylinos.cn>
---
 drivers/net/usb/usbnet.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 6a3cca104af9..c612f8f606e5 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1013,6 +1013,11 @@ int usbnet_get_link_ksettings_internal(struct net_device *net,
 	else
 		cmd->base.speed = SPEED_UNKNOWN;
 
+	/* Now we can't get link duplex without MII,
+	 * set it DUPLEX_UNKNOWN instead of default DUPLEX_HALF
+	 */
+	cmd->base.duplex = DUPLEX_UNKNOWN;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(usbnet_get_link_ksettings_internal);
-- 
2.25.1

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Andrew Lunn]

On Mon, Jul 21, 2025 at 03:10:22PM +0800, yicongsrfy@163.com wrote:
> From: Yi Cong <yicong@kylinos.cn>
> 
> CDC device don't use mdio to get link status, duplex is set
> half as default.
> 
> Now cdc_ncm can't get duplex, set it UNKNOWN instead of half
> which might actually be in an error state.

It appears that CDC has a transfer to report the link status:
usbnet_cdc_status(). There is a u32 which contains the link speed. Is
the duplex also included in this transfer?

What does the standard say about duplex? Are 1/2 duplex modes are
supported? Is it clear stated they are not supported? 1G/half not be
supported depending on the MAC/PHY pair?

	Andrew

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[yicongsrfy]

Thanks for your reply!

According to the "Universal Serial Bus Class Definitions for Communications Devices v1.2":
In Section 6.3, which describes notifications such as NetworkConnection and ConnectionSpeedChange,
there is no mention of duplex status.In particular, for ConnectionSpeedChange, its data payload
only contains two 32-bit unsigned integers, corresponding to the uplink and downlink speeds.

Since CDC has no way to obtain the duplex status of the device, ethtool displays a default
value of "Half". I think it would be better to display "unknown" instead of potentially showing
incorrect information — for example, my device is actually operating at 1Gbps Full-duplex,
but ethtool shows 1Gbps Half-duplex.

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Andrew Lunn]

On Tue, Jul 22, 2025 at 10:09:33AM +0800, yicongsrfy@163.com wrote:
> Thanks for your reply!
> 
> According to the "Universal Serial Bus Class Definitions for Communications Devices v1.2":
> In Section 6.3, which describes notifications such as NetworkConnection and ConnectionSpeedChange,
> there is no mention of duplex status.In particular, for ConnectionSpeedChange, its data payload
> only contains two 32-bit unsigned integers, corresponding to the uplink and downlink speeds.

Thanks for checking this.

Just one more question. This is kind of flipping the question on its
head. Does the standard say devices are actually allowed to support
1/2 duplex? Does it say they are not allowed to support 1/2 duplex?

If duplex is not reported, maybe it is because 1/2 duplex is simply
not allowed, so there is no need to report it.

> Since CDC has no way to obtain the duplex status of the device, ethtool displays a default
> value of "Half". I think it would be better to display "unknown" instead of potentially showing
> incorrect information — for example, my device is actually operating at 1Gbps Full-duplex,
> but ethtool shows 1Gbps Half-duplex.

I agree that reporting 1/2 is probably wrong. But we have to decide
between "unknown" and "full".

	Andrew

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[yicongsrfy]

On Tue, 22 Jul 2025 15:06:37 +0200	[thread overview] Andrew <andrew@lunn.ch> wrote:
>
> On Tue, Jul 22, 2025 at 10:09:33AM +0800, yicongsrfy@163.com wrote:
> > Thanks for your reply!
> >
> > According to the "Universal Serial Bus Class Definitions for Communications Devices v1.2":
> > In Section 6.3, which describes notifications such as NetworkConnection and ConnectionSpeedChange,
> > there is no mention of duplex status.In particular, for ConnectionSpeedChange, its data payload
> > only contains two 32-bit unsigned integers, corresponding to the uplink and downlink speeds.
>
> Thanks for checking this.
>
> Just one more question. This is kind of flipping the question on its
> head. Does the standard say devices are actually allowed to support
> 1/2 duplex? Does it say they are not allowed to support 1/2 duplex?
>
> If duplex is not reported, maybe it is because 1/2 duplex is simply
> not allowed, so there is no need to report it.
>

No, the standard does not mention anything about duplex at all.

However, Chapter 2 of the standard describes the scope of devices
covered by CDC, including wired and wireless network adapters,
among others.

We know that wireless communication is inherently half-duplex;
for wired network adapters, the duplex status depends on the
capabilities of both communication ends.

One of the USB network adapters I have (AX88179) supports both
the vendor-specific driver and the cdc_ncm driver.

When using the vendor-specific driver, all operational states
function normally, and the information reported by ethtool
matches the actual hardware behavior — which means the hardware
definitely supports both full-duplex and half-duplex modes.

The issue we are discussing only occurs when using the cdc_ncm driver.

To further investigate, I conducted the following tests using
the cdc_ncm driver:

1. I set the peer network adapter (r8169) to auto-negotiation
and connected it to the device under test. On the peer adapter,
I was able to observe that the link was operating in full-duplex mode.

2. I disabled auto-negotiation on the peer adapter and manually
set it to half-duplex. The setting was successfully applied,
and communication remained functional.

From these two tests, we can conclude that both full-duplex
and half-duplex modes are supported — the problem is simply
that the duplex status cannot be retrieved in the absence of
MII support.

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Oliver Neukum]

On 23.07.25 03:29, yicongsrfy@163.com wrote:

> No, the standard does not mention anything about duplex at all.
> 
> However, Chapter 2 of the standard describes the scope of devices
> covered by CDC, including wired and wireless network adapters,
> among others.
> 
> We know that wireless communication is inherently half-duplex;

Well, no. 802.11 is half-duplex. Cell phones are capable of full
duplex. CDC is not just network cards.

> for wired network adapters, the duplex status depends on the
> capabilities of both communication ends.

On ethernet. Again CDC is not limited to ethernet.
  
>  From these two tests, we can conclude that both full-duplex
> and half-duplex modes are supported — the problem is simply
> that the duplex status cannot be retrieved in the absence of
> MII support.

Sort of. You are asking a generic driver to apply a concept
from ethernet. It cannot. Ethernet even if it is half-duplex
is very much symmetrical in speed. Cable modems do not, just
to give an example.

I think we need to centralize the reaction to stuff that is not ethernet.

	Regards
		Oliver

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[yicongsrfy]

On Wed, 23 Jul 2025 09:17:02 +0200 Oliver <oneukum@suse.com> wrote:
>
> On 23.07.25 03:29, yicongsrfy@163.com wrote:
>
> >  From these two tests, we can conclude that both full-duplex
> > and half-duplex modes are supported — the problem is simply
> > that the duplex status cannot be retrieved in the absence of
> > MII support.
>
> Sort of. You are asking a generic driver to apply a concept
> from ethernet. It cannot. Ethernet even if it is half-duplex
> is very much symmetrical in speed. Cable modems do not, just
> to give an example.
>
> I think we need to centralize the reaction to stuff that is not ethernet.

Thanks!

I think I understand what you mean now.
You're suggesting to create a unified interface or
framework to retrieve the duplex status of all CDC
protocol-supported devices?
This seems like a rather big undertaking, and one of the key
reasons is that the CDC protocol itself does not define anything
related to duplex status — unlike the 802.3 standard, which
clearly defines how to obtain this information via MDIO.

Coming back to the issue described in this patch,
usbnet_get_link_ksettings_internal is currently only used in
cdc_ether.c and cdc_ncm.c as a callback for ethtool.
Can we assume that this part only concerns Ethernet devices
(and that, at least for now, none of the existing devices can
retrieve the duplex status through this interface)?

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Oliver Neukum]

On 23.07.25 10:44, yicongsrfy@163.com wrote:
> On Wed, 23 Jul 2025 09:17:02 +0200 Oliver <oneukum@suse.com> wrote:
>>
>> On 23.07.25 03:29, yicongsrfy@163.com wrote:
>>
>>>   From these two tests, we can conclude that both full-duplex
>>> and half-duplex modes are supported — the problem is simply
>>> that the duplex status cannot be retrieved in the absence of
>>> MII support.
>>
>> Sort of. You are asking a generic driver to apply a concept
>> from ethernet. It cannot. Ethernet even if it is half-duplex
>> is very much symmetrical in speed. Cable modems do not, just
>> to give an example.
>>
>> I think we need to centralize the reaction to stuff that is not ethernet.
> 
> Thanks!
> 
> I think I understand what you mean now.
> You're suggesting to create a unified interface or
> framework to retrieve the duplex status of all CDC
> protocol-supported devices?

Well no. We have to understand that the difference in duplex
status apply only to a subset of devices. In a way the network
layer is deficient in only having an unknown status rather than
an unknown and an inapplicable status.

If we are to retain a single status for simplicity, then
I'd say that the default being half duplex rather than
unknown is wrong.

> This seems like a rather big undertaking, and one of the key
> reasons is that the CDC protocol itself does not define anything
> related to duplex status — unlike the 802.3 standard, which
> clearly defines how to obtain this information via MDIO.

CDC is not a network protocol. CDC is a protocol for talking
to network devices. CDC can define a way to transmit duplex
information. If the protocol under CDC does not have the notion,
this will be of no use.

> Coming back to the issue described in this patch,
> usbnet_get_link_ksettings_internal is currently only used in
> cdc_ether.c and cdc_ncm.c as a callback for ethtool.
> Can we assume that this part only concerns Ethernet devices
> (and that, at least for now, none of the existing devices can
> retrieve the duplex status through this interface)?

Yes. It is not really that important. But strictly speaking the
network layer is using the wrong default.

	Regards
		Oliver

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Andrew Lunn]

> Coming back to the issue described in this patch,
> usbnet_get_link_ksettings_internal is currently only used in
> cdc_ether.c and cdc_ncm.c as a callback for ethtool.
> Can we assume that this part only concerns Ethernet devices
> (and that, at least for now, none of the existing devices can
> retrieve the duplex status through this interface)?

"ethtool" can be used with any sort of interface, if the driver
implements the op. So you should not assume it is only used for
Ethernet.

	Andrew

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Andrew Lunn]

On Wed, Jul 23, 2025 at 09:29:26AM +0800, yicongsrfy@163.com wrote:
> On Tue, 22 Jul 2025 15:06:37 +0200	[thread overview] Andrew <andrew@lunn.ch> wrote:
> >
> > On Tue, Jul 22, 2025 at 10:09:33AM +0800, yicongsrfy@163.com wrote:
> > > Thanks for your reply!
> > >
> > > According to the "Universal Serial Bus Class Definitions for Communications Devices v1.2":
> > > In Section 6.3, which describes notifications such as NetworkConnection and ConnectionSpeedChange,
> > > there is no mention of duplex status.In particular, for ConnectionSpeedChange, its data payload
> > > only contains two 32-bit unsigned integers, corresponding to the uplink and downlink speeds.
> >
> > Thanks for checking this.
> >
> > Just one more question. This is kind of flipping the question on its
> > head. Does the standard say devices are actually allowed to support
> > 1/2 duplex? Does it say they are not allowed to support 1/2 duplex?
> >
> > If duplex is not reported, maybe it is because 1/2 duplex is simply
> > not allowed, so there is no need to report it.
> >
> 
> No, the standard does not mention anything about duplex at all.

O.K. So please set duplex to unknown.

     Andrew

Re: [PATCH] usbnet: Set duplex status to unknown in the absence of MII

[Jakub Kicinski]

On Wed, 23 Jul 2025 15:05:27 +0200 Andrew Lunn wrote:
> > > Thanks for checking this.
> > >
> > > Just one more question. This is kind of flipping the question on its
> > > head. Does the standard say devices are actually allowed to support
> > > 1/2 duplex? Does it say they are not allowed to support 1/2 duplex?
> > >
> > > If duplex is not reported, maybe it is because 1/2 duplex is simply
> > > not allowed, so there is no need to report it.
> > >  
> > 
> > No, the standard does not mention anything about duplex at all.  
> 
> O.K. So please set duplex to unknown.

.. and update the commit message and the code comment to reflect 
the outcome of the discussion better.
-- 
pw-bot: cr

[PATCH v2] usbnet: Set duplex status to unknown in the absence of MII

[yicongsrfy]

From: Yi Cong <yicong@kylinos.cn>

Currently, USB CDC devices that do not use MDIO to get link status have
their duplex mode set to half-duplex by default. However, since the CDC
specification does not define a duplex status, this can be misleading.

This patch changes the default to DUPLEX_UNKNOWN in the absence of MII,
which more accurately reflects the state of the link and avoids implying
an incorrect or error state.

v2: rewrote commmit messages and code comments

Link: https://lore.kernel.org/all/20250723152151.70a8034b@kernel.org/
Signed-off-by: Yi Cong <yicong@kylinos.cn>
---
 drivers/net/usb/usbnet.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 6a3cca104af9..b870e7c6d6a0 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1013,6 +1013,13 @@ int usbnet_get_link_ksettings_internal(struct net_device *net,
 	else
 		cmd->base.speed = SPEED_UNKNOWN;
 
+	/* The standard "Universal Serial Bus Class Definitions
+	 * for Communications Devices v1.2" does not specify
+	 * anything about duplex status.
+	 * So set it DUPLEX_UNKNOWN instead of default DUPLEX_HALF.
+	 */
+	cmd->base.duplex = DUPLEX_UNKNOWN;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(usbnet_get_link_ksettings_internal);
-- 
2.25.1

Re: [PATCH v2] usbnet: Set duplex status to unknown in the absence of MII

[Oliver Neukum]

On 24.07.25 03:31, yicongsrfy@163.com wrote:
> From: Yi Cong <yicong@kylinos.cn>
> 
> Currently, USB CDC devices that do not use MDIO to get link status have
> their duplex mode set to half-duplex by default. However, since the CDC
> specification does not define a duplex status, this can be misleading.
> 
> This patch changes the default to DUPLEX_UNKNOWN in the absence of MII,
> which more accurately reflects the state of the link and avoids implying
> an incorrect or error state.
> 
> v2: rewrote commmit messages and code comments
> 
> Link: https://lore.kernel.org/all/20250723152151.70a8034b@kernel.org/
> Signed-off-by: Yi Cong <yicong@kylinos.cn>
Acked-by: Oliver Neukum <oneukum@suse.com>

Re: [PATCH v2] usbnet: Set duplex status to unknown in the absence of MII

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 24 Jul 2025 09:31:33 +0800 you wrote:
> From: Yi Cong <yicong@kylinos.cn>
> 
> Currently, USB CDC devices that do not use MDIO to get link status have
> their duplex mode set to half-duplex by default. However, since the CDC
> specification does not define a duplex status, this can be misleading.
> 
> This patch changes the default to DUPLEX_UNKNOWN in the absence of MII,
> which more accurately reflects the state of the link and avoids implying
> an incorrect or error state.
> 
> [...]

Here is the summary with links:
  - [v2] usbnet: Set duplex status to unknown in the absence of MII
    https://git.kernel.org/netdev/net-next/c/a75afcd188e1

You are awesome, thank you!
-- 
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