hw/sd/sdcard: Fix handling of disabled boot partitions

The enable bits in the EXT_CSD_PART_CONFIG ext_csd register do *not*
specify whether the boot partitions exist, but whether they are enabled
for booting. Existence of the boot partitions is specified by a
EXT_CSD_BOOT_MULT != 0.

Currently, in the case of boot-partition-size=1M and boot-config=0,
Linux detects boot partitions of 1M. But as sd_bootpart_offset always
returns 0, all reads/writes are mapped to the same offset in the backing
file.

Fix this bug by calculating the offset independent of which partition is
enabled for booting.

This bug is unlikely to affect many users with QEMU's current set of
boards, because only aspeed sets boot-partition-size, and it also
sets boot-config to 8. So to run into this a user would have to
manually mark the boot partition non-booting from within the guest.

Cc: [email protected]
Signed-off-by: Jan Luebbe <[email protected]>
Message-id: [email protected]
Reviewed-by: Peter Maydell <[email protected]>
[PMM: added note to commit message about effects of bug]
Signed-off-by: Peter Maydell <[email protected]>
(cherry picked from commit 9601076)
Signed-off-by: Michael Tokarev <[email protected]>

hw/sd/sd.c

@@ -774,19 +774,12 @@ static uint32_t sd_blk_len(SDState *sd)
  */
 static uint32_t sd_bootpart_offset(SDState *sd)
 {
-    bool partitions_enabled;
     unsigned partition_access;
 
     if (!sd->boot_part_size || !sd_is_emmc(sd)) {
         return 0;
     }
 
-    partitions_enabled = sd->ext_csd[EXT_CSD_PART_CONFIG]
-                                   & EXT_CSD_PART_CONFIG_EN_MASK;
-    if (!partitions_enabled) {
-        return 0;
-    }
-
     partition_access = sd->ext_csd[EXT_CSD_PART_CONFIG]
                                  & EXT_CSD_PART_CONFIG_ACC_MASK;
     switch (partition_access) {