mmc: host: add note that set_ios needs to handle 0Hz properly

[linux-2.6-block.git] / include / linux / mmc / host.h

diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 8dd4d290ab0d8608b9596c18da8db1096715badb..85800b48241fad5599f20793831b3e8e1dd7178f 100644 (file)
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -93,28 +93,39 @@ struct mmc_host_ops {
        void    (*pre_req)(struct mmc_host *host, struct mmc_request *req,
                           bool is_first_req);
        void    (*request)(struct mmc_host *host, struct mmc_request *req);
+
+       /*
+        * Avoid calling the next three functions too often or in a "fast
+        * path", since underlaying controller might implement them in an
+        * expensive and/or slow way. Also note that these functions might
+        * sleep, so don't call them in the atomic contexts!
+        */
+
+       /*
+        * Notes to the set_ios callback:
+        * ios->clock might be 0. For some controllers, setting 0Hz
+        * as any other frequency works. However, some controllers
+        * explicitly need to disable the clock. Otherwise e.g. voltage
+        * switching might fail because the SDCLK is not really quiet.
+        */
+       void    (*set_ios)(struct mmc_host *host, struct mmc_ios *ios);
+
        /*
-        * Avoid calling these three functions too often or in a "fast path",
-        * since underlaying controller might implement them in an expensive
-        * and/or slow way.
-        *
-        * Also note that these functions might sleep, so don't call them
-        * in the atomic contexts!
-        *
         * Return values for the get_ro callback should be:
         *   0 for a read/write card
         *   1 for a read-only card
         *   -ENOSYS when not supported (equal to NULL callback)
         *   or a negative errno value when something bad happened
-        *
+        */
+       int     (*get_ro)(struct mmc_host *host);
+
+       /*
         * Return values for the get_cd callback should be:
         *   0 for a absent card
         *   1 for a present card
         *   -ENOSYS when not supported (equal to NULL callback)
         *   or a negative errno value when something bad happened
         */
-       void    (*set_ios)(struct mmc_host *host, struct mmc_ios *ios);
-       int     (*get_ro)(struct mmc_host *host);
        int     (*get_cd)(struct mmc_host *host);
 
        void    (*enable_sdio_irq)(struct mmc_host *host, int enable);