ARM: qcom: scm: Flush the command buffer only instead of the entire cache

scm_call flushes the entire cache before calling into the secure world.
This is both a performance penalty as well as insufficient on SMP systems
where the CPUs possess a write-back L1 cache. Flush only the command and
response buffers instead, moving the responsibility of flushing any other
cached buffer (being passed to the secure world) to callers.

Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kumar Gala <galak@codeaurora.org>

arch/arm/mach-qcom/scm.c

@@ -196,12 +196,12 @@ static int __scm_call(const struct scm_command *cmd)
 	u32 cmd_addr = virt_to_phys(cmd);
 
 	/*
-	 * Flush the entire cache here so callers don't have to remember
-	 * to flush the cache when passing physical addresses to the secure
-	 * side in the buffer.
+	 * Flush the command buffer so that the secure world sees
+	 * the correct data.
 	 */
-	flush_cache_all();
-	outer_flush_all();
+	__cpuc_flush_dcache_area((void *)cmd, cmd->len);
+	outer_flush_range(cmd_addr, cmd_addr + cmd->len);
+
 	ret = smc(cmd_addr);
 	if (ret < 0)
 		ret = scm_remap_error(ret);
@@ -238,6 +238,13 @@ static void scm_inv_range(unsigned long start, unsigned long end)
  * @resp_len: length of the response buffer
  *
  * Sends a command to the SCM and waits for the command to finish processing.
+ *
+ * A note on cache maintenance:
+ * Note that any buffers that are expected to be accessed by the secure world
+ * must be flushed before invoking scm_call and invalidated in the cache
+ * immediately after scm_call returns. Cache maintenance on the command and
+ * response buffers is taken care of by scm_call; however, callers are
+ * responsible for any other cached buffers passed over to the secure world.
  */
 int scm_call(u32 svc_id, u32 cmd_id, const void *cmd_buf, size_t cmd_len,
 		void *resp_buf, size_t resp_len)