[PATCH u-boot] mmc: spl: Add comments for default_spl_mmc_emmc_boot_partition()

Pali Rohár
2023-04-04T20:30:12Z

Add documentation what is default_spl_mmc_emmc_boot_partition() function
doing, what each return value from this function means and why there is
magic conversion from 7 to 0. Also add FIXME comment for future fixes.

All information is based on the JEDEC eMMC JESD84-A441 specification.

Signed-off-by: Pali Rohár <pali@kernel.org>

---
This patch depends on another patch:
mmc: Use EXT_CSD_EXTRACT_BOOT_PART() macro for extracting boot part
https://patchwork.ozlabs.org/project/uboot/patch/20230311104427.18421-1-pali@kernel.org/
---
common/spl/spl_mmc.c | 42 +++++++++++++++++++++++++++++++++++++++---
1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/common/spl/spl_mmc.c b/common/spl/spl_mmc.c
index a0722167044f..2426500dbcb9 100644
--- a/common/spl/spl_mmc.c
+++ b/common/spl/spl_mmc.c
@@ -370,13 +370,49 @@ unsigned long __weak spl_mmc_get_uboot_raw_sector(struct mmc *mmc,
int default_spl_mmc_emmc_boot_partition(struct mmc *mmc)
{
int part;
+ /*
+ * This function returns the eMMC partition from which
+ * would be loaded next payload (e.g. U-Boot proper).
+ * This partition will be set into eMMC partition access
+ * register and therefore return value of this function
+ * has same meaning as EXT_CSD_EXTRACT_PARTITION_ACCESS.
+ *
+ * Meaning of EXT_CSD_EXTRACT_PARTITION_ACCESS (3-bit) is:
+ * 0 - User area
+ * 1 - Boot partition 1
+ * 2 - Boot partition 2
+ * 3 - Replay Protected Memory Block (RPMB)
+ * 4 - General Purpose partition 1
+ * 5 - General Purpose partition 2
+ * 6 - General Purpose partition 3
+ * 7 - General Purpose partition 4
+ */
#ifdef CONFIG_SYS_MMCSD_RAW_MODE_EMMC_BOOT_PARTITION
part = CONFIG_SYS_MMCSD_RAW_MODE_EMMC_BOOT_PARTITION;
#else
/*
- * We need to check what the partition is configured to.
- * 1 and 2 match up to boot0 / boot1 and 7 is user data
- * which is the first physical partition (0).
+ * When explicit CONFIG_SYS_MMCSD_RAW_MODE_EMMC_BOOT_PARTITION
+ * config option was not chosen during build time we choose
+ * the boot partition based on the eMMC boot partition enable
+ * register (EXT_CSD_EXTRACT_BOOT_PART).
+ *
+ * Meaning of EXT_CSD_EXTRACT_BOOT_PART (3-bit) value is:
+ * 0 - Booting from this eMMC device is disabled
+ * 1 - Boot partition 1 is used for booting
+ * 2 - Boot partition 2 is used for booting
+ * 3 - Reserved
+ * 4 - Reserved
+ * 5 - Reserved
+ * 6 - Reserved
+ * 7 - User area is used for booting
+ *
+ * Note: See difference between EXT_CSD_EXTRACT_PARTITION_ACCESS
+ * and EXT_CSD_EXTRACT_BOOT_PART, specially about User area value.
+ *
+ * FIXME: When booting from this eMMC device is explicitly
+ * disabled then we use User area for booting. This is incorrect.
+ * Probably we should skip this eMMC device and select the next
+ * one for booting. Or at least throw warning about this fallback.
*/
part = EXT_CSD_EXTRACT_BOOT_PART(mmc->part_config);
if (part == 7)
--
2.20.1