serdev_device_write_buf() returns negative numbers on errors.  When
the return value compares to unsigned integer `len`, it promotes to
quite large positive number.

Fix it.

Fixes: 04a8bdd135cc ("platform/chrome: cros_ec_uart: Add transport layer")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Link: https://lore.kernel.org/r/20230109081554.3792547-1-tzungbi@kernel.org

serdev_device_set_client_ops() is called before `ec_dev` is fully
initialized.  This can result in cros_ec_uart_rx_bytes() being called
while `ec_dev` is still not initialized, resulting in a kernel panic.

Call serdev_device_set_client_ops() after `ec_dev` is initialized.

Fixes: 04a8bdd135cc ("platform/chrome: cros_ec_uart: Add transport layer")
Signed-off-by: Robert Zieba <robertzieba@google.com>
[tzungbi: modified commit message and fixed context conflict.]
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20221229094738.2304044-1-tzungbi@kernel.org

Existing firmware uses the "PRP0001" _HID and an associated compatible
string to enumerate the cros_ec_uart.

Add DT enumeration support for already shipped firmware.

Signed-off-by: Bhanu Prakash Maiya <bhanumaiya@chromium.org>
Co-developed-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Mark Hasemeyer <markhas@chromium.org>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20221227123212.v13.3.Ie23c217d69ff25d7354db942613f143bbc8ef891@changeid

This patch does following:
1. Adds a new cros-ec-uart driver. This driver can send EC requests on
   UART and process response packets received on UART transport.
2. Once probed, this driver will initialize the serdev device based on
   the underlying information in the ACPI resource. After serdev device
   properties are set, this driver will register itself cros-ec.
3. High level driver can use this implementation to talk to ChromeOS
   Embedded Controller device in case it supports UART as transport.
4. When cros-ec driver initiates a request packet, outgoing message is
   processed in buffer and sent via serdev. Once bytes are sent, driver
   enables a wait_queue.
5. Since ChromeOS EC device sends response asynchronously, AP's TTY
   driver accumulates response bytes and calls the registered callback.
   TTY driver can send multiple callback for bytes ranging from 1 to MAX
   bytes supported by EC device.
6. Driver waits for EC_MSG_DEADLINE_MS to collect and process received
   bytes. It wakes wait_queue if expected bytes are received or else
   wait_queue timeout. Based on the error condition, driver returns
   data_len or error to cros_ec.

Signed-off-by: Bhanu Prakash Maiya <bhanumaiya@chromium.org>
Co-developed-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Mark Hasemeyer <markhas@chromium.org>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20221227123212.v13.1.If7926fcbad397bc6990dd725690229bed403948c@changeid