[PATCH v3 1/2] clocksource: timer-econet-en751221: fix timer block mapping at boot

[kpursoty]

timer_init() used DIV_ROUND_UP(num_possible_cpus(), 2) to determine how
many register blocks to iomap. At early boot with VPE-based SMP, MIPS
reports num_possible_cpus()=1 (VPEs not yet brought online), giving
num_blocks=1. Only membase[0] is then mapped via of_iomap.

The EN751627 SoC has 2 physical cores, each with 2 VPEs, giving NR_CPUS=4
and two timer register blocks (one per core). Each block serves two VPEs:
block 0 handles CPU0+CPU1 (core 0), block 1 handles CPU2+CPU3 (core 1).
The block count is a silicon constant: DIV_ROUND_UP(NR_CPUS, 2) = 2.

cevt_init() calls cevt_dev_init(i) for each possible CPU via
for_each_possible_cpu(). On EN7528/EN751627 with 4 VPEs, NR_CPUS=4 so
cevt_dev_init(2) is called. cevt_dev_init(2) writes to reg_compare(2)
which dereferences membase[2>>1] = membase[1], which is NULL:

  CPU 0 Unable to handle kernel paging request at virtual address 00000008
  epc : iowrite32+0x4/0x10
  ra  : cevt_dev_init+0x40/0x64

Fix: replace the runtime calculation with ECONET_NUM_BLOCKS, which is
DIV_ROUND_UP(NR_CPUS, 2) evaluated at compile time. This is the same
expression used to declare the membase[] array, so the loop bound and
array size are provably consistent. For NR_CPUS=4 this is always 2,
correctly mapping both register blocks regardless of how many VPEs are
visible at early boot.

Fixes: 3b4c33ac87d0 ("clocksource/drivers: Add EcoNet Timer HPT driver")
Signed-off-by: Kervin Pursoty <kpursoty@xxxxxxxxx>
Reviewed-by: Caleb James DeLisle <cjd@xxxxxxxx>
---
v3: No changes. Resent as v3 to keep version in sync with 2/2.
v2: Add Reviewed-by from Caleb James DeLisle.

 drivers/clocksource/timer-econet-en751221.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/clocksource/timer-econet-en751221.c b/drivers/clocksource/timer-econet-en751221.c
--- a/drivers/clocksource/timer-econet-en751221.c
+++ b/drivers/clocksource/timer-econet-en751221.c
@@ -160,5 +160,4 @@ static u64 notrace sched_clock_read(void)
 static int __init timer_init(struct device_node *np)
 {
-	int num_blocks = DIV_ROUND_UP(num_possible_cpus(), 2);
 	struct clk *clk;
 	int ret;

@@ -172,5 +171,5 @@ static int __init timer_init(struct device_node *np)

 	econet_timer.freq_hz = clk_get_rate(clk);

-	for (int i = 0; i < num_blocks; i++) {
+	for (int i = 0; i < ECONET_NUM_BLOCKS; i++) {
 		econet_timer.membase[i] = of_iomap(np, i);
 		if (!econet_timer.membase[i]) {
 			pr_err("%pOFn: failed to map register [%d]\n", np, i);