mfd: cros_ec: Use a zero-length array for command data

Commit 1b84f2a4cd4a ("mfd: cros_ec: Use fixed size arrays to transfer
data with the EC") modified the struct cros_ec_command fields to not
use pointers for the input and output buffers and use fixed length
arrays instead.

This change was made because the cros_ec ioctl API uses that struct
cros_ec_command to allow user-space to send commands to the EC and
to get data from the EC. So using pointers made the API not 64-bit
safe. Unfortunately this approach was not flexible enough for all
the use-cases since there may be a need to send larger commands
on newer versions of the EC command protocol.

So to avoid to choose a constant length that it may be too big for
most commands and thus wasting memory and CPU cycles on copy from
and to user-space or having a size that is too small for some big
commands, use a zero-length array that is both 64-bit safe and
flexible. The same buffer is used for both output and input data
so the maximum of these values should be used to allocate it.

Suggested-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/linux/mfd/cros_ec.h b/include/linux/mfd/cros_ec.h
index 14cf522123dd..7eee38abd02a 100644
--- a/include/linux/mfd/cros_ec.h
+++ b/include/linux/mfd/cros_ec.h
@@ -42,8 +42,7 @@ enum {
  * @outsize: Outgoing length in bytes
  * @insize: Max number of bytes to accept from EC
  * @result: EC's response to the command (separate from communication failure)
- * @outdata: Outgoing data to EC
- * @indata: Where to put the incoming data from EC
+ * @data: Where to put the incoming data from EC and outgoing data to EC
  */
 struct cros_ec_command {
         uint32_t version;
@@ -51,8 +50,7 @@ struct cros_ec_command {
         uint32_t outsize;
         uint32_t insize;
         uint32_t result;
-        uint8_t outdata[EC_PROTO2_MAX_PARAM_SIZE];
-        uint8_t indata[EC_PROTO2_MAX_PARAM_SIZE];
+        uint8_t data[0];
 };
 
 /**