netdev - Re: [PATCH] net: usb: lan78xx: Enforce a minimum interrupt polling period

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Message-ID: <83b8ec69-0f7b-4ede-bf4f-f35b5d4fa4b2@gmx.de>
Date: Tue, 11 Mar 2025 15:36:42 +0100
From: Fiona Klute <fiona.klute@....de>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, Thangaraj Samynathan <Thangaraj.S@...rochip.com>,
 Rengarajan Sundararajan <Rengarajan.S@...rochip.com>,
 UNGLinuxDriver@...rochip.com, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, linux-usb@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-list@...pberrypi.com,
 stable@...r.kernel.org
Subject: Re: [PATCH] net: usb: lan78xx: Enforce a minimum interrupt polling
 period

Am 11.03.25 um 14:22 schrieb Andrew Lunn:
> On Tue, Mar 11, 2025 at 01:30:54PM +0100, Fiona Klute wrote:
>> Am 10.03.25 um 22:27 schrieb Andrew Lunn:
>>> On Mon, Mar 10, 2025 at 05:59:31PM +0100, Fiona Klute wrote:
>>>> If a new reset event appears before the previous one has been
>>>> processed, the device can get stuck into a reset loop. This happens
>>>> rarely, but blocks the device when it does, and floods the log with
>>>> messages like the following:
>>>>
>>>>     lan78xx 2-3:1.0 enp1s0u3: kevent 4 may have been dropped
>>>>
>>>> The only bit that the driver pays attention to in the interrupt data
>>>> is "link was reset". If there's a flapping status bit in that endpoint
>>>> data (such as if PHY negotiation needs a few tries to get a stable
>>>> link), polling at a slower rate allows the state to settle.
>>>
>>> Could you expand on this a little bit more. What is the issue you are
>>> seeing?
>>
>> What happens is that *sometimes* when the interface is activated (up, im
>> my case via NetworkManager) during boot, the "kevent 4 may have been
>> dropped" message starts to be emitted about every 6 or 7 ms.
>
> This sounding a bit like an interrupt storm. The PHY interrupt is not
> being cleared correctly. PHY interrupts are level interrupts, so if
> you don't clear the interrupt at the source, it will fire again as
> soon as you re-enable it.
>
> So which PHY driver is being used? If you look for the first kernel
> message about the lan78xx it probably tells you.
>
>> [   27.918335] Call trace:
>> [   27.918338]  console_flush_all+0x2b0/0x4f8 (P)
>> [   27.918346]  console_unlock+0x8c/0x170
>> [   27.918352]  vprintk_emit+0x238/0x3b8
>> [   27.918357]  dev_vprintk_emit+0xe4/0x1b8
>> [   27.918364]  dev_printk_emit+0x64/0x98
>> [   27.918368]  __netdev_printk+0xc8/0x228
>> [   27.918376]  netdev_info+0x70/0xa8
>> [   27.918382]  phy_print_status+0xcc/0x138
>> [   27.918386]  lan78xx_link_status_change+0x78/0xb0
>> [   27.918392]  phy_link_change+0x38/0x70
>> [   27.918398]  phy_check_link_status+0xa8/0x110
>> [   27.918405]  _phy_start_aneg+0x5c/0xb8
>> [   27.918409]  lan88xx_link_change_notify+0x5c/0x128
>> [   27.918416]  _phy_state_machine+0x12c/0x2b0
>> [   27.918420]  phy_state_machine+0x34/0x80
>> [   27.918425]  process_one_work+0x150/0x3b8
>> [   27.918432]  worker_thread+0x2a4/0x4b8
>> [   27.918438]  kthread+0xec/0xf8
>> [   27.918442]  ret_from_fork+0x10/0x20
>> [   27.918534] lan78xx 2-3:1.0 enp1s0u3: kevent 4 may have been dropped
>> [   27.924985] lan78xx 2-3:1.0 enp1s0u3: kevent 4 may have been dropped
>
> Ah, O.K. This tells me the PHY is a lan88xx. And there is a workaround
> involved for an issue in this PHY. Often PHYs are driven by polling
> for status changes once per second. Not all PHYs/boards support
> interrupts. It could be this workaround has only been tested with
> polling, not interrupts, and so is broken when interrupts are used.
>
> As a quick hack test, in lan78xx_phy_init()
>
> 	/* if phyirq is not set, use polling mode in phylib */
> 	if (dev->domain_data.phyirq > 0)
> 		phydev->irq = dev->domain_data.phyirq;
> 	else
> 		phydev->irq = PHY_POLL;
>
> Hard code phydev->irq to PHY_POLL, so interrupts are not used.
>
> See if you can reproduce the issue when interrupts are not used.
Thank you, I'll test that. Given the issue appears rarely it'll
unfortunately take a while to be (mostly) sure.

Best regards,
Fiona