[PATCH v10 RESEND 05/15] arm_mpam: Don't stall MPAM enable when an MSC fails to probe

[Jinjie Ruan]

mpam_msc_drv_probe() registers the discovery cpuhp callbacks only when
the number of successfully probed MSCs reaches fw_num_msc. Every failure
path returns before the counter is incremented, and a failed probe is
final - the driver core does not retry. One failing MSC therefore leaves
the counter below fw_num_msc forever: the discovery callbacks are never
registered, mpam_enable() is never scheduled, and MPAM stays dead
system-wide - the resctrl filesystem is never registered, and no error
message hints at the cause.

An MSC can legitimately fail to probe on an otherwise working system:
firmware may describe MSCs that are not accessible from any possible
CPU, e.g. when the possible CPUs are restricted with nr_cpus=. The MSCs
of the clipped CPUs then fail with "MSC is not accessible from any
CPU!", or their cache resources become unresolvable from PPTT.

Count every completed probe, successful or not. Failed MSCs are not on
mpam_all_msc (either never added, or removed by mpam_msc_drv_remove()),
so the discovery path skips them naturally, and MPAM is enabled for the
surviving devices.

Cc: stable@xxxxxxxxxxxxxxx
Cc: Ben Horgan <ben.horgan@xxxxxxx>
Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: Fenghua Yu <fenghuay@xxxxxxxxxx>
Cc: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Fixes: f04046f2577a5 ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 drivers/resctrl/mpam_devices.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index 61cbc75625fa..a44d66a5733d 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c
@@ -2120,21 +2120,31 @@ static int mpam_msc_drv_probe(struct platform_device *pdev)
 	msc = do_mpam_msc_drv_probe(pdev);
 	mutex_unlock(&mpam_list_lock);
 
-	if (IS_ERR(msc))
-		return PTR_ERR(msc);
+	if (IS_ERR(msc)) {
+		err = PTR_ERR(msc);
+		goto out;
+	}
 
 	/* Create RIS entries described by firmware */
 	err = acpi_mpam_parse_resources(msc, plat_data);
-	if (err) {
+	if (err)
 		mpam_msc_drv_remove(pdev);
-		return err;
-	}
-
-	if (atomic_add_return(1, &mpam_num_msc) == fw_num_msc)
+out:
+	/*
+	 * A failed MSC must not prevent MPAM being enabled for the
+	 * surviving devices. Firmware may describe MSCs that are not
+	 * accessible from any possible CPU, e.g. when the possible CPUs
+	 * were restricted by nr_cpus=. The failed device is not
+	 * re-probed, so count it here too: otherwise the counter never
+	 * reaches fw_num_msc, the discovery cpuhp callbacks are never
+	 * registered and mpam_enable() is never scheduled - MPAM stays
+	 * dead system-wide, without any error message.
+	 */
+	if (atomic_inc_return(&mpam_num_msc) == fw_num_msc)
 		mpam_register_cpuhp_callbacks(mpam_discovery_cpu_online, NULL,
 					      "mpam:drv_probe");
 
-	return 0;
+	return err;
 }
 
 static struct platform_driver mpam_msc_driver = {
-- 
2.34.1