[PATCH v2 7/7] riscv: Add some comments to start.S

Sean Anderson
2020-09-14T14:23:03Z

This adds comments regarding the ordering and purpose of certain
instructions as I understand them.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
---

Changes in v2:
- Clarify comments regarding tp

arch/riscv/cpu/start.S | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index a16af79fbe..cb1347559c 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -43,7 +43,10 @@ _start:
csrr a0, CSR_MHARTID
#endif

- /* save hart id and dtb pointer */
+ /*
+ * Save hart id and dtb pointer. The thread pointer register is not
+ * modified by C code. It is used by secondary_hart_loop.
+ */
mv tp, a0
mv s1, a1

@@ -54,10 +57,18 @@ _start:
*/
mv gp, zero

+ /*
+ * Set the trap handler. This must happen after initializing gp because
+ * the handler may use it.
+ */
la t0, trap_entry
csrw MODE_PREFIX(tvec), t0

- /* mask all interrupts */
+ /*
+ * Mask all interrupts. Interrupts are disabled globally (in m/sstatus)
+ * for U-Boot, but we will need to read m/sip to determine if we get an
+ * IPI
+ */
csrw MODE_PREFIX(ie), zero

#if CONFIG_IS_ENABLED(SMP)
@@ -410,6 +421,10 @@ secondary_hart_relocate:
mv gp, a2
#endif

+/*
+ * Interrupts are disabled globally, but they can still be read from m/sip. The
+ * wfi function will wake us up if we get an IPI, even if we do not trap.
+ */
secondary_hart_loop:
wfi

--
2.28.0