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>

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 45ea968..0b4c2ed 100644 (file)
--- 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 4c4bddf..340cc0c 100644 (file)
--- 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);