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>

2 files changed, 9 insertions, 0 deletions

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 45ea968e7dd1..0b4c2ed22bce 100644
--- a/drivers/mmc/core/core.c
+++ b/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;
diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 4c4bddf5ef61..340cc0c9409f 100644
--- a/include/linux/mmc/host.h
+++ b/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);