Re: [PATCH] arm64: dts: imx8mp-debix-model-a: Disable EEE for 1000T

[Emanuele Ghidoli <ghidoliemanuele@gmail.com>]

On 27/10/2025 10:00, Russell King (Oracle) wrote:
> On Mon, Oct 27, 2025 at 09:47:53AM +0100, Emanuele Ghidoli wrote:
>> On 27/10/2025 08:27, Laurent Pinchart wrote:
>>> Hi Andrew,
>>>
>>> Thank you for your quick reply.
>>>
>>> On Mon, Oct 27, 2025 at 04:08:42AM +0100, Andrew Lunn wrote:
>>>> Adding Russell King
>>>>
>>>> On Sun, Oct 26, 2025 at 02:29:04PM +0200, Laurent Pinchart wrote:
>>>>> Energy Efficient Ethernet (EEE) is broken at least for 1000T on the EQOS
>>>>> (DWMAC) interface. When connected to an EEE-enabled peer, the ethernet
>>>>> devices produces an interrupts storm. Disable EEE support to fix it.
>>>>>
>>>>> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>>>>> ---
>>>>> The exact reason for the interrupt storm is unknown, and my attempts to
>>>>> diagnose it was hindered by my lack of expertise with DWMAC. As far as I
>>>>> understand, the DWMAC implements EEE support, and so does the RTL8211E
>>>>> PHY according to its datasheet.
>>>>
>>>> I believe for DWMAC it is a synthesis option. However, there is a bit
>>>> indicating if the hardware supports it.
>>>>
>>>> The PHY should not be able to trigger an interrupt storm in the
>>>> MAC. So this is likely to be an DWMAC issue.
>>>>
>>>> Which interrupt bit is causing the storm?
>>>
>>> That's where I hit my first wall :-)
>>>
>>> I've tried to diagnose the issue by adding interrupt counters to
>>> dwmac4_irq_status(), counting interrupts for each bit of GMAC_INT_STATUS
>>> (0x00b0). Bit RGSMIIIS (0) is the only one that seems linked to the
>>> interrupts storm, increasing at around 10k per second. However, the
>>> corresponding bit in GMAC_INT_EN (0x00b4) is *not* set.
>>>
>>> The ENET_EQOS interrupt on the i.MX8MP is an OR'ed signal that combines
>>> four interrupt sources:
>>>
>>> - ENET QOS TSN LPI RX exit Interrupt
>>> - ENET QOS TSN Host System Interrupt
>>> - ENET QOS TSN Host System RX Channel Interrupts
>>> - ENET QOS TSN Host System TX Channel Interrupts
>>>
>>> The last two interrupt sources are themselves local OR of channels[4:0].
>>>
>>> I ould suspect that the LPI RX exit interrupt is the one that fires
>>> constantly given its name, but I'm not sure how to test that.
>>>
>>>>> What each side does exactly is unknown
>>>>> to me. One theory I've heard to explain the issue is that the two
>>>>> implementations conflict. There is no register in the RTL8211E PHY to
>>>>> disable EEE on the PHY side while still advertising its support to the
>>>>> peer and relying on the implementation in the DWMAC (if this even makes
>>>>> sense)
>>>>
>>>> It does not make sense. EEE is split into two major parts. The two
>>>> PHYs communicate with each other to negotiate the feature, if both
>>>> ends support it and both ends want to use it. The result of this
>>>> negotiation is then passed to the MACs.
>>>>
>>>> It is then the MAC who decides when to send a Low Power Indication to
>>>> the PHY to tell the PHY to enter low power mode. The MAC also wakes
>>>> the PHY when it has packets to send.
>>>>
>>>> A quick look at the data sheet for the RTL8211E suggests this is what
>>>> is supports.
>>>>
>>>> There are a few PHYs which implement SmartEEE, or some other similar
>>>> name. They operate differently, the PHY does it all, and the MAC is
>>>> not even aware EEE is happening. Such PHYs should really only be
>>>> paired with MACs which do not support EEE. An EEE capable MAC paired
>>>> with a SmartEEE PHY could have problems, but hopefully the EEE
>>>> abilities and negotiation registers in the PHY would be sufficient to
>>>> dissuade the MAC from doing EEE. But i would not expect a setup like
>>>> this to trigger an interrupt storm.
>>>
>>> Thanks for the explanation, I read documents to try and figure out how
>>> it worked and didn't find such a clear and concise high-level summary.
>>>
>>> I'm not very experienced with ethernet, but I can easily test patches or
>>> even rough ideas on hardware.
>>>
>>
>> Hi Laurent,
>> I had the same problem, interrupt storm plus link instability with dwmac.
> 
> You never said that in your patch description. You said "it causes
> link instability and communication failures." Have you investigated
> what the cause of the interrupt storm is?
> 
>> I found out that 2c81f3357136 ("net: stmmac: convert to phylink PCS support")
>> commit is the one causing the problem to me.
The correct commit is 4218647d4556 (“net: stmmac: convert to phylink managed
EEE support”).>
> You claim this commit enables EEE by default. It does. However, stmmac
> _before_ this commit enables EEE by default as I've already explained,
> quoting the old code which effects this. I've asked you to test
> further. So far, I've heard nothing back.
> 
> What has changed is that we no longer do anything with the RGSMIIS
> status, and in theory keep the mask/enable for this disabled. Howeer,
> that is a subsequent commit.
> 
Hi Russell,

Sorry, I made a copy-and-paste mistake earlier.


I identified it through a bisect, and reverting this commit (or disabling EEE)
resolves the issue I’m seeing.

I’m continuing to investigate further to understand the root cause.