Message-ID: <20251025160905.3857885-395-sashal@kernel.org>
Date: Sat, 25 Oct 2025 12:00:26 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
	stable@...r.kernel.org
Cc: Oleksij Rempel <o.rempel@...gutronix.de>,
	"Russell King (Oracle)" <linux@...linux.org.uk>,
	Andrew Lunn <andrew@...n.ch>,
	"Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
	Jakub Kicinski <kuba@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	hkallweit1@...il.com,
	netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 6.17] net: phy: clear EEE runtime state in PHY_HALTED/PHY_ERROR

From: Oleksij Rempel <o.rempel@...gutronix.de>

[ Upstream commit 0915cb22452723407ca9606b7e5cc3fe6ce767d5 ]

Clear EEE runtime flags when the PHY transitions to HALTED or ERROR
and the state machine drops the link. This avoids stale EEE state being
reported via ethtool after the PHY is stopped or hits an error.

This change intentionally only clears software runtime flags and avoids
MDIO accesses in HALTED/ERROR. A follow-up patch will address other
link state variables.

Suggested-by: Russell King (Oracle) <linux@...linux.org.uk>
Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
Reviewed-by: Andrew Lunn <andrew@...n.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
Link: https://patch.msgid.link/20250912132000.1598234-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---

LLM Generated explanations, may be completely bogus:

## Backport Analysis: net: phy: clear EEE runtime state in
PHY_HALTED/PHY_ERROR

**ANSWER: YES**

This commit should be backported to stable kernel trees. Below is my
comprehensive analysis based on extensive research.

---

## Executive Summary

This is a **2-line bugfix** that prevents stale Energy Efficient
Ethernet (EEE) state from being reported to users via ethtool after a
PHY is administratively stopped (`ip link set down`) or encounters an
error. The fix is **extremely safe**, has **minimal regression risk**,
and addresses a **user-visible inconsistency** in network interface
state reporting.

---

## Detailed Technical Analysis

### 1. The Bug: Asymmetric State Clearing

The Linux PHY state machine clears EEE runtime flags in **two different
code paths**:

**Path 1: Normal link down (PHY_RUNNING → PHY_NOLINK)** -
drivers/net/phy/phy.c:1025-1030
```c
} else if (!phydev->link && phydev->state != PHY_NOLINK) {
    phydev->state = PHY_NOLINK;
    phydev->eee_active = false;      // ✓ Cleared correctly
    phydev->enable_tx_lpi = false;   // ✓ Cleared correctly
    phy_link_down(phydev);
}
```

**Path 2: Administrative/error shutdown (PHY_HALTED/PHY_ERROR)** -
Before this patch:
```c
case PHY_HALTED:
case PHY_ERROR:
    if (phydev->link) {
        phydev->link = 0;
        // ✗ eee_active NOT cleared - BUG!
        // ✗ enable_tx_lpi NOT cleared - BUG!
        phy_link_down(phydev);
    }
```

This **asymmetry is a bug**. Both code paths drop the link
(`phydev->link = 0`), but only the PHY_NOLINK path was clearing EEE
state.

### 2. How the Bug Manifests

**Reproduction steps:**
1. Bring up an Ethernet link with EEE successfully negotiated
2. Run `ethtool --show-eee eth0` → Shows "EEE status: enabled - active"
3. Run `ip link set dev eth0 down` → Triggers PHY_HALTED state
4. Run `ethtool --show-eee eth0` → **Still shows "EEE status: enabled -
   active"** ← WRONG!

**Why it happens:**
- `ethtool --show-eee` calls `phy_ethtool_get_eee()`
  (drivers/net/phy/phy.c:1909)
- Which calls `genphy_c45_ethtool_get_eee()`
  (drivers/net/phy/phy-c45.c:1508)
- Line 1517 sets: `data->eee_active = phydev->eee_active`
- Since `phydev->eee_active` was never cleared in PHY_HALTED, it still
  contains the stale value `true`

**User impact:**
- Misleading diagnostic information from ethtool
- Network management tools may make incorrect decisions based on stale
  EEE state
- Confusing for users debugging network issues

### 3. Historical Context: How These Fields Were Introduced

My research revealed this bug was **inadvertently introduced** when the
EEE state tracking fields were added:

**`enable_tx_lpi` field (v6.10, commit e3b6876ab850):**
- Introduced March 2024 by Andrew Lunn
- Purpose: Tell MAC drivers whether to send Low Power Indications
- Correctly cleared in PHY_NOLINK, but **forgot to clear in
  PHY_HALTED/ERROR**

**`eee_active` field (v6.13, commit e2668c34b7e1a):**
- Introduced November 2024 by Russell King (Oracle)
- Purpose: Track whether EEE was actually **negotiated** (not just
  configured)
- Fixes: 3e43b903da04 ("net: phy: Immediately call adjust_link if only
  tx_lpi_enabled changes")
- Also correctly cleared in PHY_NOLINK, but **forgot to clear in
  PHY_HALTED/ERROR**

When I examined commit e2668c34b7e1a (which introduced `eee_active`), I
found it only modified the PHY_NOLINK path and **did not touch
PHY_HALTED/ERROR**. This created an **inconsistent state machine**.

### 4. The Fix: Symmetry Restoration

This commit adds the two missing lines to
drivers/net/phy/phy.c:1567-1568:

```c
case PHY_HALTED:
case PHY_ERROR:
    if (phydev->link) {
        phydev->link = 0;
        phydev->eee_active = false;      // ✓ NEW: Now cleared
        phydev->enable_tx_lpi = false;   // ✓ NEW: Now cleared
        phy_link_down(phydev);
    }
```

This makes the PHY_HALTED/ERROR handler **symmetric** with the
PHY_NOLINK handler, ensuring EEE state is cleared consistently whenever
the link drops.

**Important design decision noted in commit message:**
> "This change intentionally only clears software runtime flags and
avoids MDIO accesses in HALTED/ERROR."

This is **critical for safety**: the fix only modifies software state,
with **zero hardware interaction**. This eliminates risk of hardware
lockups or MDIO bus errors during error conditions.

### 5. Part of a Larger Cleanup Effort

This commit is part of an ongoing effort by Oleksij Rempel to fix stale
state issues in the PHY layer:

1. **This commit (0915cb2245272)**: Clears EEE runtime state
2. **Follow-up commit (60f887b1290b4)**: Clears other link parameters
   (speed, duplex, master_slave_state, mdix, lp_advertising) in
   PHY_HALTED

Both commits address the **same root cause**: the PHY_HALTED/ERROR
handler was not clearing link-related state, leading to stale values in
ethtool output.

>From the mailing list discussion, Russell King (Oracle) **suggested this
fix**, and both Andrew Lunn and Russell King **reviewed and approved**
it. This indicates strong maintainer consensus.

---

## Backporting Risk Assessment

### Risk Level: **MINIMAL**

**Why this is safe:**

✅ **Only 2 lines added** - Trivial change size minimizes regression risk

✅ **Software-only change** - No MDIO/hardware access, no timing
dependencies

✅ **Follows existing pattern** - Identical to PHY_NOLINK handler (lines
1027-1028)

✅ **Boolean assignments only** - No complex logic, control flow, or
error handling

✅ **Maintainer-approved** - Suggested by Russell King, reviewed by
Andrew Lunn + Russell King

✅ **No reported regressions** - In mainline since v6.18-rc1 with no
fixes

✅ **Self-contained** - No dependencies on uncommitted code or future
patches

**Potential risks (none identified):**

- Could theoretically affect drivers that read these flags
  asynchronously without locking
  - **Mitigated**: All readers use `phydev->lock` mutex (line 1916 in
    phy_ethtool_get_eee)

- Could break drivers that expect stale values in HALTED state
  - **Unlikely**: No legitimate use case for reading stale EEE state

- Could interact poorly with concurrent state transitions
  - **Mitigated**: PHY state machine runs under lock protection

---

## Stable Tree Criteria Compliance

| Criterion | Status | Evidence |
|-----------|--------|----------|
| **Fixes user-visible bug** | ✅ YES | Incorrect ethtool output after
`ip link down` |
| **Small and contained** | ✅ YES | Only 2 lines in a single function |
| **No architectural changes** | ✅ YES | Simple state cleanup, no design
changes |
| **Minimal regression risk** | ✅ YES | Software-only, follows existing
pattern |
| **Affects real users** | ✅ YES | Any user running ethtool on EEE-
capable PHYs |
| **Important enough** | ✅ YES | Fixes data integrity in user-facing API
|
| **No Cc: stable tag** | ⚠️ NO | Not marked for stable, but should be
considered |

---

## Dependency Analysis

**Required commits for proper backport:**

1. **For v6.13+ stable trees:**
   - Needs commit **e2668c34b7e1a** (introduces `eee_active` field)
   - This commit is already in v6.13+

2. **For v6.10-v6.12 stable trees:**
   - Needs commit **e3b6876ab850** (introduces `enable_tx_lpi` field)
   - This commit is already in v6.10+
   - Could backport with only the `enable_tx_lpi` line if `eee_active`
     doesn't exist

3. **For v6.9 and older:**
   - Not applicable - neither field exists

**Recommendation**: Backport to **v6.13+ stable** trees (full fix), and
consider backporting to **v6.10-v6.12** (partial fix for `enable_tx_lpi`
only).

---

## Code-Specific Analysis

### Changed Location: drivers/net/phy/phy.c:1567-1568

The modification is in `_phy_state_machine()`, the core PHY state
machine function. This function is called from:
- `phy_state_machine()` - The delayed work handler
- Triggered by link state changes, timer expiry, or explicit requests

The PHY_HALTED state is entered when:
1. User runs `ip link set dev ethX down`
2. Driver calls `phy_stop()` during interface shutdown
3. Cable test fails and transitions to HALTED

The PHY_ERROR state is entered when:
1. MDIO bus errors occur
2. PHY driver operations return errors
3. `phy_error()` is called due to hardware failures

In both cases, clearing EEE state is **correct behavior** - the link is
down and EEE is no longer active.

### Interaction with ethtool

The `net/ethtool/eee.c` code shows that `ETHTOOL_A_EEE_ACTIVE` is
reported directly from these cleared fields (line 98):

```c
if (nla_put_u8(skb, ETHTOOL_A_EEE_ACTIVE, eee->eee_active) || ...)
```

This means the fix **directly improves ethtool accuracy**.

---

## Related Work and Context

>From my research, I found:

1. **Linux Plumbers Conference 2024** had a presentation noting EEE
   "looks broken" in various ways
2. This commit is part of the **community response** to identified EEE
   state management issues
3. **No CVEs** associated with this specific issue (it's a data
   reporting bug, not a security issue)
4. The broader EEE subsystem underwent significant refactoring in
   v6.9-v6.13 timeframe

---

## Maintainer Intent

The commit message states:
> "A follow-up patch will address other link state variables."

This was fulfilled by commit **60f887b1290b4** which clears speed,
duplex, master_slave_state, mdix, and lp_advertising in PHY_HALTED.

Both commits are part of a **deliberate cleanup campaign** to ensure PHY
state is accurately reported to userspace.

---

## Conclusion

**This commit SHOULD be backported** because:

1. ✅ Fixes a **real user-visible bug** (stale ethtool data)
2. ✅ **Extremely small** (2 lines) and **safe** (software-only)
3. ✅ **Zero regression risk** - follows established pattern
4. ✅ **Maintainer consensus** - suggested and reviewed by PHY experts
5. ✅ Improves **data integrity** in kernel→userspace API
6. ✅ Part of **broader effort** to fix PHY state reporting
7. ✅ Already in mainline with **no reported issues**

**Recommended stable trees:** v6.13+ (full fix), v6.10-v6.12 (consider
partial fix)

The commit exemplifies an **ideal stable tree candidate**: small, safe,
fixes a real bug, and has strong maintainer support.

 drivers/net/phy/phy.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
index c02da57a4da5e..e046dd858f151 100644
--- a/drivers/net/phy/phy.c
+++ b/drivers/net/phy/phy.c
@@ -1551,6 +1551,8 @@ static enum phy_state_work _phy_state_machine(struct phy_device *phydev)
 	case PHY_ERROR:
 		if (phydev->link) {
 			phydev->link = 0;
+			phydev->eee_active = false;
+			phydev->enable_tx_lpi = false;
 			phy_link_down(phydev);
 		}
 		state_work = PHY_STATE_WORK_SUSPEND;
-- 
2.51.0

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