mmc: core: clarify how to use post_req in case of errors

The err condition in post_req() is set to undo a call made to pre_req()
that hasn't been started yet.  The err condition is not set if an MMC
request returns an error.
Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/core.c

@@ -320,8 +320,14 @@ struct mmc_async_req *mmc_start_req(struct mmc_host *host,
 		mmc_wait_for_req_done(host, host->areq->mrq);
 		err = host->areq->err_check(host->card, host->areq);
 		if (err) {
+			/* post process the completed failed request */
 			mmc_post_req(host, host->areq->mrq, 0);
 			if (areq)
+				/*
+				 * Cancel the new prepared request, because
+				 * it can't run until the failed
+				 * request has been properly handled.
+				 */
 				mmc_post_req(host, areq->mrq, -EINVAL);
 
 			host->areq = NULL;

include/linux/mmc/host.h

@@ -109,6 +109,9 @@ struct mmc_host_ops {
 	 * It is optional for the host to implement pre_req and post_req in
 	 * order to support double buffering of requests (prepare one
 	 * request while another request is active).
+	 * pre_req() must always be followed by a post_req().
+	 * To undo a call made to pre_req(), call post_req() with
+	 * a nonzero err condition.
 	 */
 	void	(*post_req)(struct mmc_host *host, struct mmc_request *req,
 			    int err);