[PATCH] arm_mpam: resctrl: Leave CDP off when there are too few PARTIDs

Enabling CDP requires at least 2 PARTIDs. If there are too few PARTIDs,
resctrl_arch_set_cdp_enabled() returns -EINVAL, but only after it has
recorded CDP as enabled, halved the monitor count and turned off the
mbw_max MBA control. resctrl then believes CDP is off, which leaves the
two sides out of step and the halved monitor count and the missing MBA
control are not undone.

Check the PARTID limit before anything is changed so a failed enable has
no effect.

Fixes: 6789fb99282c ("arm_mpam: resctrl: Add CDP emulation")
Signed-off-by: Jinjie Ruan <redacted>
---
RFC v2 -> v1:
- Check mpam_partid_max ahead and The partid_d/partid_i determination
  remains in place.
- Update the commit message as Ben suggested.
---
 drivers/resctrl/mpam_resctrl.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 5468367776e2..aa3b3a093857 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c

@@ -210,6 +210,9 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
 	if (enable)
 		pr_warn("CDP is an expert feature and may cause MPAM to malfunction.\n");
 
+	if (enable && mpam_partid_max < 1)
+		return -EINVAL;
+
 	/*
 	 * resctrl_arch_set_cdp_enabled() is only called with enable set to
 	 * false on error and unmount.

@@ -239,9 +242,6 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
 		mpam_resctrl_controls[RDT_RESOURCE_MBA].resctrl_res.alloc_capable = true;
 
 	if (enable) {
-		if (mpam_partid_max < 1)
-			return -EINVAL;
-
 		partid_d = resctrl_get_config_index(RESCTRL_RESERVED_CLOSID, CDP_DATA);
 		partid_i = resctrl_get_config_index(RESCTRL_RESERVED_CLOSID, CDP_CODE);
 	}

-- 
2.34.1