[PATCH 3/4] platform/chrome: cros_ec_proto: update cros_ec_check_result() comment

[Tzung-Bi Shih <tzungbi@kernel.org>]

At first glance, cros_ec_check_result() is quite like cros_ec_map_error().
They check for `ec_msg->result` and return corresponding errors.  However,
as calling from `pkt_xfer` and `cmd_xfer`, cros_ec_check_result() should
not report furthermore errors.  -EAGAIN is the only exception.

See [1][2][3] for some known userland programs' code.  The return code
from ioctl only denotes the EC communication status.  Userland programs
would further analyze the `result` in struct cros_ec_command* for
follow-up actions (e.g. [4]).

To clarify, update the function comment.

[1]: https://crrev.com/54400e93a75ef440a83d6eaac2cec066daf99cf0/util/comm-dev.c#154
[2]: https://crrev.com/fe32670a89bf59e1aff84bba9dd3295657b85e9b/cros_ec_dev.c#296
[3]: https://crrev.com/4e19eb1d89de0422ff1bbd3f7260b131c761098c/drivers/google/cros_ec_dev.c#120
[4]: https://crrev.com/54400e93a75ef440a83d6eaac2cec066daf99cf0/util/comm-dev.c#164

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
---
Changes from previous version:
(https://patchwork.kernel.org/project/chrome-platform/patch/20220517082817.1591526-1-tzungbi@kernel.org/)
- Update the link of [3].

 drivers/platform/chrome/cros_ec_proto.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/platform/chrome/cros_ec_proto.c b/drivers/platform/chrome/cros_ec_proto.c
index 01ab58b3269b..13ced9d2dd71 100644
--- a/drivers/platform/chrome/cros_ec_proto.c
+++ b/drivers/platform/chrome/cros_ec_proto.c
@@ -204,9 +204,12 @@ EXPORT_SYMBOL(cros_ec_prepare_tx);
  * @msg: Message to check.
  *
  * This is used by ChromeOS EC drivers to check the ec_msg->result for
- * errors and to warn about them.
+ * EC_RES_IN_PROGRESS and to warn about them.
  *
- * Return: 0 on success or negative error code.
+ * The function should not check for furthermore error codes.  Otherwise,
+ * it would break the ABI.
+ *
+ * Return: -EAGAIN if ec_msg->result == EC_RES_IN_PROGRESS.  Otherwise, 0.
  */
 int cros_ec_check_result(struct cros_ec_device *ec_dev,
 			 struct cros_ec_command *msg)
-- 
2.36.0.550.gb090851708-goog