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doc: fix r8127_ptp.c Patch 2 description — spinlock deadlock

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The previous description said to simply uncomment rtl8127_set_local_time()
inside rtl8127_hwtstamp_enable(). That causes an RCU stall because
_rtl8127_phc_settime() tries to re-acquire phy_lock, which is already held
by the caller. The correct fix is to move the call to after the spinlock is
released. Update the patch description accordingly.

Assisted-by: Claude Sonnet 4.6
Signed-off-by: Raymond Knopp <raymond.knopp@eurecom.fr>
This commit is contained in:
Raymond Knopp
2026-06-03 08:03:19 +00:00
parent 5257d49203
commit d15ea06da3
1 changed files with 26 additions and 4 deletions
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30
doc/dragonwing-build.md
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@@ -615,16 +615,38 @@ make KERNELDIR=~/linux-6.18.12 \
> + hrtimer_setup(&tp->pps_timer, rtl8127_hrtimer_for_pps, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> ```
>
> **Patch 2 — PHC initialisation (`rtl8127_set_local_time` commented out).**
> Without this, the PHC starts at 0 after driver reset while the PTP master uses
> Unix time, causing the delay/offset formulas to produce garbage values (~1.8 s rms).
> Around line 642, uncomment the call:
> **Patch 2 — PHC initialisation (`rtl8127_set_local_time` called inside spinlock).**
> Without initialising the PHC, it starts at 0 after driver reset while the PTP
> master uses Unix time, causing delay/offset formulas to produce garbage (~1.8 s rms).
> The vendor source has the call commented out; uncommenting it naively causes an RCU
> stall / spinlock deadlock because `_rtl8127_phc_settime` tries to re-acquire
> `phy_lock`, which is already held by `rtl8127_hwtstamp_enable`.
> The fix is to remove the call from inside the locked section and re-add it after
> the spinlock is released. In `rtl8127_hwtstamp_enable` (around line 629):
> ```diff
> if (enable) {
> //trx timestamp interrupt enable
> rtl8127_set_eth_phy_ocp_bit(tp, PTP_INER, BIT_2 | BIT_3);
> //set isr clear mode
> rtl8127_set_eth_phy_ocp_bit(tp, PTP_GEN_CFG, BIT_0);
> //clear ptp isr
> rtl8127_mdio_direct_write_phy_ocp(tp, PTP_INSR, 0xFFFF);
> //enable ptp
> rtl8127_ptp_enable_config(tp);
>
> - //rtl8127_set_local_time(tp);
> } else {
> ```
> and add the call after `r8127_spin_unlock`:
> ```diff
> r8127_spin_unlock(&tp->phy_lock, flags);
>
> + /* _rtl8127_phc_settime() acquires phy_lock internally; call after unlock */
> + if (enable)
> + rtl8127_set_local_time(tp);
> +
> return 0;
> }
> ```
If option A modules are already loaded, unload them first: