sunxi: eMMC: Improve automatic boot source detection

When the Allwinner BROM loads the SPL from an eMMC boot partition, it
sets the boot source byte to the same value as when booting from the
user data partition. This prevents us from determining the boot source
to load U-Boot proper from the proper partition for sure.

The generic SPL MMC code already looks at the enabled boot partition
number, to load U-Boot proper from the same partition, but this fails
if there is nothing bootable in this partition, as the BROM then
silently falls back to the user data partition, which the SPL misses.

To learn about the actual boot source anyway, we repeat the algorithm
the BROM used to select the boot partition in the first place:
- Test EXT_CSD[179] to check if an eMMC boot partition is enabled.
- Test EXT_CSD[177] to check for valid MMC interface settings.
- Check if BOOT_ACK is enabled.
- Check the beginning of the first sector for a valid eGON signature.
- Load the whole SPL.
- Recalculate the checksum to verify the SPL is valid.

If one of those steps fails, we bail out and continue loading from the
user data partition. Otherwise we load from the selected boot partition.

Since the boot source is needed twice in the boot process, we cache the
result of this test to avoid doing this costly test multiple times.

This allows the very same image file to be put onto an SD card, into the
eMMC user data partition or into the eMMC boot partition, and safely
loads the whole of U-Boot from there.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

arch/arm/mach-sunxi/board.c

@@ -346,6 +346,89 @@ __weak void sunxi_sram_init(void)
 {
 }
 
+/*
+ * When booting from an eMMC boot partition, the SPL puts the same boot
+ * source code into SRAM A1 as when loading the SPL from the normal
+ * eMMC user data partition: 0x2. So to know where we have been loaded
+ * from, we repeat the BROM algorithm here: checking for a valid eGON boot
+ * image at offset 0 of a (potentially) selected boot partition.
+ * If any of the conditions is not met, it must have been the eMMC user
+ * data partition.
+ */
+static bool sunxi_valid_emmc_boot(struct mmc *mmc)
+{
+	struct blk_desc *bd = mmc_get_blk_desc(mmc);
+	uint32_t *buffer = (void *)(uintptr_t)CONFIG_SYS_TEXT_BASE;
+	struct boot_file_head *egon_head = (void *)buffer;
+	int bootpart = EXT_CSD_EXTRACT_BOOT_PART(mmc->part_config);
+	uint32_t spl_size, emmc_checksum, chksum = 0;
+	ulong count;
+
+	/* The BROM requires BOOT_ACK to be enabled. */
+	if (!EXT_CSD_EXTRACT_BOOT_ACK(mmc->part_config))
+		return false;
+
+	/*
+	 * The BOOT_BUS_CONDITION register must be 4-bit SDR, with (0x09)
+	 * or without (0x01) high speed timings.
+	 */
+	if ((mmc->ext_csd[EXT_CSD_BOOT_BUS_WIDTH] & 0x1b) != 0x01 &&
+	    (mmc->ext_csd[EXT_CSD_BOOT_BUS_WIDTH] & 0x1b) != 0x09)
+		return false;
+
+	/* Partition 0 is the user data partition, bootpart must be 1 or 2. */
+	if (bootpart != 1 && bootpart != 2)
+		return false;
+
+	/* Failure to switch to the boot partition is fatal. */
+	if (mmc_switch_part(mmc, bootpart))
+		return false;
+
+	/* Read the first block to do some sanity checks on the eGON header. */
+	count = blk_dread(bd, 0, 1, buffer);
+	if (count != 1 || !sunxi_egon_valid(egon_head))
+		return false;
+
+	/* Read the rest of the SPL now we know it's halfway sane. */
+	spl_size = buffer[4];
+	count = blk_dread(bd, 1, DIV_ROUND_UP(spl_size, bd->blksz) - 1,
+			  buffer + bd->blksz / 4);
+
+	/* Save the checksum and replace it with the "stamp value". */
+	emmc_checksum = buffer[3];
+	buffer[3] = 0x5f0a6c39;
+
+	/* The checksum is a simple ignore-carry addition of all words. */
+	for (count = 0; count < spl_size / 4; count++)
+		chksum += buffer[count];
+
+	debug("eMMC boot part SPL checksum: stored: 0x%08x, computed: 0x%08x\n",
+	       emmc_checksum, chksum);
+
+	return emmc_checksum == chksum;
+}
+
+u32 spl_mmc_boot_mode(struct mmc *mmc, const u32 boot_device)
+{
+	static u32 result = ~0;
+
+	if (result != ~0)
+		return result;
+
+	result = MMCSD_MODE_RAW;
+	if (!IS_SD(mmc) && IS_ENABLED(CONFIG_SUPPORT_EMMC_BOOT)) {
+		if (sunxi_valid_emmc_boot(mmc))
+			result = MMCSD_MODE_EMMCBOOT;
+		else
+			mmc_switch_part(mmc, 0);
+	}
+
+	debug("%s(): %s part\n", __func__,
+	      result == MMCSD_MODE_RAW ? "user" : "boot");
+
+	return result;
+}
+
 void board_init_f(ulong dummy)
 {
 	sunxi_sram_init();