mmc: sd: add fall-back extension register parsing attempt

Several types of cards have a common bug where the vendor did not
populate the General Information block, but extension registers are
present at assumed function offsets, and the features are correctly
implemented.

Probe the offsets to see if valid data is returned.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

Author: P33M

drivers/mmc/core/sd.c

@@ -1218,6 +1218,26 @@ static int mmc_sd_read_ext_regs(struct mmc_card *card)
 		goto out;
 	}
 
+	/* Some cards have zeroes in GEN_INFO but correctly implement EXT_PERF and EXT_PWR */
+	if (!memcmp(card->ext_reg_buf, gen_info_buf, 512)) {
+		pr_info("%s: using fall-back extension register parsing\n",
+			mmc_hostname(card->host));
+		/* PWR typically hard-coded at FNO=1 */
+		err = sd_parse_ext_reg_power(card, 1, 0, 0);
+		if (err) {
+			pr_err("%s: error %d parsing SD Power extension\n",
+				mmc_hostname(card->host), err);
+			goto out;
+		}
+		/* PERF typically hard-coded at FNO=2 */
+		err = sd_parse_ext_reg_perf(card, 2, 0, 0);
+		if (err) {
+			pr_err("%s: error %d parsing SD Performance extension\n",
+			       mmc_hostname(card->host), err);
+		}
+		goto out;
+	}
+
 	/* General info structure revision. */
 	memcpy(&rev, &gen_info_buf[0], 2);