ARM: 7445/1: mm: update CONTEXTIDR register to contain PID of current process

This patch introduces a new Kconfig option which, when enabled, causes
the kernel to write the PID of the current task into the PROCID field
of the CONTEXTIDR on context switch. This is useful when analysing
hardware trace, since writes to this register can be configured to emit
an event into the trace stream.

The thread notifier for writing the PID is deliberately kept separate
from the ASID-writing code so that we can support newer processors using
LPAE, where the ASID is stored in TTBR0. As such, the switch_mm code is
updated to perform a read-modify-write sequence to ensure that we don't
clobber the PID on CPUs using the classic 2-level page tables.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/arm/mm/proc-v6.S b/arch/arm/mm/proc-v6.S
index 5900cd520e84..86b8b480634f 100644
--- a/arch/arm/mm/proc-v6.S
+++ b/arch/arm/mm/proc-v6.S
@@ -107,6 +107,12 @@ ENTRY(cpu_v6_switch_mm)
         mcr     p15, 0, r2, c7, c5, 6           @ flush BTAC/BTB
         mcr     p15, 0, r2, c7, c10, 4          @ drain write buffer
         mcr     p15, 0, r0, c2, c0, 0           @ set TTB 0
+#ifdef CONFIG_PID_IN_CONTEXTIDR
+        mrc     p15, 0, r2, c13, c0, 1          @ read current context ID
+        bic     r2, r2, #0xff                   @ extract the PID
+        and     r1, r1, #0xff
+        orr     r1, r1, r2                      @ insert into new context ID
+#endif
         mcr     p15, 0, r1, c13, c0, 1          @ set context ID
 #endif
         mov     pc, lr