ata: libahci: re-enable FBS after error handling on a port multiplier

ahci_do_softreset() disables FBS before resetting a device behind a port multiplier, as required by AHCI 1.2 section 9.3.9, and re-enables it only when that softreset succeeds. If it fails, for example because a hot-plugged drive is still spinning up ("softreset failed (device not ready)"), EH escalates to a hardreset of the PMP link, and nothing turns FBS back on: only ahci_pmp_attach() does, which runs at the next boot. The port keeps working with command-based switching until then.

Seen on a Marvell 88SE9235 (1b4b:9235) with Marvell 88SM9705 port multipliers (1b4b:9705, in a Synology DX1222 expansion unit): after hot-plugging a drive, the kernel logged "FBS is disabled" followed by "softreset failed (device not ready)" and a PMP hardreset, and never "FBS is enabled" again (twice, on two different ports).

At the end of ahci_error_handler(), re-enable FBS if a PMP is still attached and FBS is supported but disabled.

Tested on 7.2.6 with the patch built as an out-of-tree libahci (the stock ahci module binds to it, symbol CRCs are unchanged). Hot-plugging a drive into a free port of the multiplier then logged "FBS is disabled", "softreset failed (device not ready)" and a PMP hardreset, followed by "re-enabling FBS after error handling" and "FBS is enabled". Afterwards two other drives behind the same multiplier read 259 + 259 MB/s in parallel (O_DIRECT, 4 MiB blocks), against 268 MB/s for one drive alone, so their commands overlap again.

---

Notes:
v2: no code changes. The only change is my author address: v1 went out on 2026-10-10 from my previous address. Please apply this version instead of v1.

The controller is passed through (VFIO) to a KVM guest; the tested kernel is the guest's. The two earlier occurrences without the patch were on ata8 and ata10 of the same controller; this test was on ata7.

 drivers/ata/libahci.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

index 08ab56e9556..6f374fd79f5 100644
@@ -2233,6 +2233,23 @@ void ahci_error_handler(struct ata_port *ap)
 
 	sata_pmp_error_handler(ap);
 
+	/*
+	 * ahci_do_softreset() turns FBS off before resetting a fan-out link
+	 * and turns it back on only if that reset succeeds. When it fails
+	 * (e.g. a hot-plugged drive is still spinning up), EH escalates to a
+	 * PMP hardreset, which never re-enables FBS, so the port would keep
+	 * command-based switching until the PMP is attached again (reboot).
+	 * Restore FBS once recovery is done and the PMP is still attached.
+	 */
+	if (sata_pmp_attached(ap) && ata_dev_enabled(ap->link.device)) {
+		struct ahci_port_priv *pp = ap->private_data;
+
+		if (pp->fbs_supported && !pp->fbs_enabled) {
+			ata_port_info(ap, "re-enabling FBS after error handling\n");
+			ahci_enable_fbs(ap);
+		}
+	}
+
 	if (!ata_dev_enabled(ap->link.device))
 		hpriv->stop_engine(ap);
 }

2.43.0