mmc: core: Default to generic_cmd6_time as timeout in __mmc_switch()

All callers of __mmc_switch() should now be specifying a valid timeout for
the CMD6 command. However, just to be sure, let's print a warning and
default to use the generic_cmd6_time in case the provided timeout_ms
argument is zero.

In this context, let's also simplify some of the corresponding code and
clarify some related comments.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200122142747.5690-4-ulf.hansson@linaro.org

drivers/mmc/core/mmc_ops.c

@@ -460,10 +460,6 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
 	bool expired = false;
 	bool busy = false;
 
-	/* We have an unspecified cmd timeout, use the fallback value. */
-	if (!timeout_ms)
-		timeout_ms = MMC_OPS_TIMEOUT_MS;
-
 	/*
 	 * In cases when not allowed to poll by using CMD13 or because we aren't
 	 * capable of polling by using ->card_busy(), then rely on waiting the
@@ -536,14 +532,19 @@ int __mmc_switch(struct mmc_card *card, u8 set, u8 index, u8 value,
 
 	mmc_retune_hold(host);
 
+	if (!timeout_ms) {
+		pr_warn("%s: unspecified timeout for CMD6 - use generic\n",
+			mmc_hostname(host));
+		timeout_ms = card->ext_csd.generic_cmd6_time;
+	}
+
 	/*
-	 * If the cmd timeout and the max_busy_timeout of the host are both
-	 * specified, let's validate them. A failure means we need to prevent
-	 * the host from doing hw busy detection, which is done by converting
-	 * to a R1 response instead of a R1B.
+	 * If the max_busy_timeout of the host is specified, make sure it's
+	 * enough to fit the used timeout_ms. In case it's not, let's instruct
+	 * the host to avoid HW busy detection, by converting to a R1 response
+	 * instead of a R1B.
 	 */
-	if (timeout_ms && host->max_busy_timeout &&
-		(timeout_ms > host->max_busy_timeout))
+	if (host->max_busy_timeout && (timeout_ms > host->max_busy_timeout))
 		use_r1b_resp = false;
 
 	cmd.opcode = MMC_SWITCH;
@@ -554,10 +555,6 @@ int __mmc_switch(struct mmc_card *card, u8 set, u8 index, u8 value,
 	cmd.flags = MMC_CMD_AC;
 	if (use_r1b_resp) {
 		cmd.flags |= MMC_RSP_SPI_R1B | MMC_RSP_R1B;
-		/*
-		 * A busy_timeout of zero means the host can decide to use
-		 * whatever value it finds suitable.
-		 */
 		cmd.busy_timeout = timeout_ms;
 	} else {
 		cmd.flags |= MMC_RSP_SPI_R1 | MMC_RSP_R1;