ARM: proc-v7: Retry uncached stmia if necessary

A failure of some CPU cores to come online has been traced to the
failure of a stm instruction while the cache is disabled. The symptom
is that the saved values read back as zeroes, a catastrophic error since
one of the values is a return address.

This patch forces a readback and retry until the correct value is
returned,

Notes:

At this stage in the boot process the core is running with its cache
disabled. Before enabling the cache its contents must be explicitly
invalidated, a process that requires quite a few registers that the
caller must preserve. Evidence suggests that something is writing a
block of zeroes over that space at a time when all other cores should
be idle, possibly some kind of write-combiner, and retrying is an
attempt to avoid the problem.

The previous attempted fix (forcing the accesses to only be 4-byte
aligned) appears to have only worked for a while and likely for less
obvious reasons such as a change in code alignment.

See: https://github.com/Hexxeh/rpi-firmware/issues/232

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

diff --git a/arch/arm/mm/proc-v7.S b/arch/arm/mm/proc-v7.S
index 28c9d32fa99a58081150ae7599569d21969a0e58..950ceb970470b3795ca0f6b5072d43bbf51d48b0 100644 (file)
--- a/arch/arm/mm/proc-v7.S
+++ b/arch/arm/mm/proc-v7.S
@@ -288,7 +288,11 @@ __v7_ca17mp_setup:
 1:     adr     r0, __v7_setup_stack_ptr
        ldr     r12, [r0]
        add     r12, r12, r0                    @ the local stack
+1:
        stmia   r12, {r1-r6, lr}                @ v7_invalidate_l1 touches r0-r6
+       ldr     r0, [r12, #(6 * 4)]             @ read back the return address
+       teq     r0, lr                          @ confirm it is correct
+       bne     1b                              @ retrying if not
        bl      v7_invalidate_l1
        ldmia   r12, {r1-r6, lr}
 #ifdef CONFIG_SMP
@@ -474,7 +478,11 @@ __v7_setup:
        adr     r0, __v7_setup_stack_ptr
        ldr     r12, [r0]
        add     r12, r12, r0                    @ the local stack
+1:
        stmia   r12, {r1-r6, lr}                @ v7_invalidate_l1 touches r0-r6
+       ldr     r0, [r12, #(6 * 4)]             @ read back the return address
+       teq     r0, lr                          @ confirm it is correct
+       bne     1b                              @ retrying if not
        bl      v7_invalidate_l1
        ldmia   r12, {r1-r6, lr}