[Skiboot] [RESEND][PATCH] phb4: Don't probe a PHB if its garded

[Vaibhav Jain]

Presently phb4_probe_stack() causes an exception while trying to probe
a PHB if its garded. This causes skiboot to go into a reboot loop with
following exception log:

 ***********************************************
 Fatal MCE at 000000003006ecd4   .probe_phb4+0x570
 CFAR : 00000000300b98a0
 <snip>
 Aborting!
CPU 0018 Backtrace:
 S: 0000000031cc37e0 R: 000000003001a51c   ._abort+0x4c
 S: 0000000031cc3860 R: 0000000030028170   .exception_entry+0x180
 S: 0000000031cc3a40 R: 0000000000001f10 *
 S: 0000000031cc3c20 R: 000000003006ecb0   .probe_phb4+0x54c
 S: 0000000031cc3e30 R: 0000000030014ca4   .main_cpu_entry+0x5b0
 S: 0000000031cc3f00 R: 0000000030002700   boot_entry+0x1b8

This is caused as phb4_probe_stack() will ignore all xscom read/write
errors to enable PHB Bars and then tries to perform an mmio to read
PHB Version registers that cause the fatal MCE.

We fix this by ignoring the PHB probe if the first xscom_write() to
populate the PHB Bar register fails, which indicates that there is
something wrong with the PHB.

Cc: stable
Fixes: dc21b4db3a2e('hw/phb4: Add initial support')
Reviewed-by: Oliver O'Halloran <oohall at gmail.com>
Signed-off-by: Vaibhav Jain <vaibhav at linux.ibm.com>
---
Change-log:

Resend -> Marked the patch to stable [Oliver]
---
 hw/phb4.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/hw/phb4.c b/hw/phb4.c
index d1245dce..89b0b859 100644
--- a/hw/phb4.c
+++ b/hw/phb4.c
@@ -5546,6 +5546,7 @@ static void phb4_probe_stack(struct dt_node *stk_node, uint32_t pec_index,
 	char *path;
 	uint64_t capp_ucode_base;
 	unsigned int max_link_speed;
+	int rc;
 
 	gcid = dt_get_chip_id(stk_node);
 	stk_index = dt_prop_get_u32(stk_node, "reg");
@@ -5567,9 +5568,17 @@ static void phb4_probe_stack(struct dt_node *stk_node, uint32_t pec_index,
 
 	/* Initialize PHB register BAR */
 	phys_map_get(gcid, PHB4_REG_SPC, phb_num, &phb_bar, NULL);
-	xscom_write(gcid, nest_stack + XPEC_NEST_STK_PHB_REG_BAR, phb_bar << 8);
-	bar_en |= XPEC_NEST_STK_BAR_EN_PHB;
+	rc = xscom_write(gcid, nest_stack + XPEC_NEST_STK_PHB_REG_BAR,
+			 phb_bar << 8);
+
+	/* A scom error here probably indicates a defective/garded PHB */
+	if (rc != OPAL_SUCCESS) {
+		prerror("PHB[%d:%d] Unable to set PHB BAR. Error=%d\n",
+		      gcid, phb_num, rc);
+		return;
+	}
 
+	bar_en |= XPEC_NEST_STK_BAR_EN_PHB;
 
 	/* Same with INT BAR (ESB) */
 	phys_map_get(gcid, PHB4_XIVE_ESB, phb_num, &irq_bar, NULL);
-- 
2.17.1