Add some notes on how the boot-flow goes while I still remember it.
Signed-off-by: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
---
arch/arm/boards/nxp-imx8mq-evk/lowlevel.c | 41 +++++++++++++++++++++--
1 file changed, 39 insertions(+), 2 deletions(-)
diff --git a/arch/arm/boards/nxp-imx8mq-evk/lowlevel.c b/arch/arm/boards/nxp-imx8mq-evk/lowlevel.c
index db746bb94..fdc964ac9 100644
--- a/arch/arm/boards/nxp-imx8mq-evk/lowlevel.c
+++ b/arch/arm/boards/nxp-imx8mq-evk/lowlevel.c
@@ -66,6 +66,26 @@ static void nxp_imx8mq_evk_sram_setup(void)
BUG_ON(ret);
}
+/*
+ * Power-on execution flow of start_nxp_imx8mq_evk() might not be
+ * obvious for a very frist read, so here's, hopefully helpful,
+ * summary:
+ *
+ * 1. MaskROM uploads PBL into OCRAM and that's where this function is
+ * executed for the first time
+ *
+ * 2. DDR is initialized and full i.MX image is loaded to the
+ * beginning of RAM
+ *
+ * 3. start_nxp_imx8mq_evk, now in RAM, is executed again
+ *
+ * 4. BL31 blob is uploaded to OCRAM and the control is transfer to it
+ *
+ * 5. BL31 exits EL3 into EL2 at address MX8MQ_ATF_BL33_BASE_ADDR,
+ * executing start_nxp_imx8mq_evk() the third time
+ *
+ * 6. Standard barebox boot flow continues
+ */
ENTRY_FUNCTION(start_nxp_imx8mq_evk, r0, r1, r2)
{
arm_cpu_lowlevel_init();
@@ -73,9 +93,23 @@ ENTRY_FUNCTION(start_nxp_imx8mq_evk, r0, r1, r2)
if (IS_ENABLED(CONFIG_DEBUG_LL))
setup_uart();
- if (get_pc() < MX8MQ_DDR_CSD1_BASE_ADDR)
+ if (get_pc() < MX8MQ_DDR_CSD1_BASE_ADDR) {
+ /*
+ * We assume that we were just loaded by MaskROM into
+ * SRAM if we are not running from DDR. We also assume
+ * that means DDR needs to be initialized for the
+ * first time.
+ */
nxp_imx8mq_evk_sram_setup();
-
+ }
+ /*
+ * Straight from the power-on we are at EL3, so the following
+ * code _will_ load and jump to ATF.
+ *
+ * However when we are re-executed upon exit from ATF's
+ * initialization routine, it is EL2 which means we'll skip
+ * loadting ATF blob again
+ */
if (current_el() == 3) {
const u8 *bl31;
size_t bl31_size;
@@ -84,6 +118,9 @@ ENTRY_FUNCTION(start_nxp_imx8mq_evk, r0, r1, r2)
imx8mq_atf_load_bl31(bl31, bl31_size);
}
+ /*
+ * Standard entry we hit once we initialized both DDR and ATF
+ */
barebox_arm_entry(MX8MQ_DDR_CSD1_BASE_ADDR,
SZ_2G + SZ_1G, __dtb_imx8mq_evk_start);
}
--
2.17.1
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