Make solo half of split keyboards (more) usable. (#13523)

* Make solo half of split keyboards (more) usable.

Using only one half of a split keyboard (that's using the split_common
framework to communicate) is not a great experience, since several read
timeouts per scan cycle cause an unusably slow scan rate.

This change blocks all split communication attempts for 500 ms
(configurable) after an error occurs, causing the scan rate to become at
least _more_ usable, but might need some tweaking to work fully on most
keyboards. One read timeout still needs to occur after the 500 ms has
passed, and if that timeout isn't low enough, some scan cycles may still
be too slow.

* Fix lint complaint.

* Require 25 consecutive comm errors to see comms as disconnected.

The number of max errors can be overridden by defining
`SPLIT_MAX_CONNECTION_ERRORS`.

* Add comments to new defines, and ability to disable disconnection check.

Also increase `SPLIT_MAX_CONNECTION_ERRORS` to 40, since it's divisible
by most relevant numbers for the description.

* Make lint happy ...again

* Only update `connection_check_timer` when needed.

* Add new defines to split keyboard documentation.

* Move connection timeout logic to transport.c, add `is_transport_connected`.

* Use split_common disconnection logic in matrix.c.

Instead of doing more or less the same thing twice.

* Move disconnection logic to `transport_master`.

Is a cleaner implementation, and causes the scan rate while disconnected
to increase instead of decrease.

* Lint fixes.

* Lower default `SERIAL_USART_TIMEOUT` to 20 ms.

The read timeout must be low enough to not cause exessively long scan
cycles when using a solo split half. 10 ms was determined from testing
to work fine even with the slowest defined baudrate of 19200 (5 ms was
too low for that case), so 20 ms should be fine for most cases.

* Remove `SERIAL_USART_TIMEOUT` from ergodox_infinity/config.h

Was somewhat mistakenly included in an earlier PR.

* Fix building with `USE_I2C`.

* Reduce built firmware size.

Not really sure why this works, the idea was taken from tzarc's work on
split disconnection.

* Tweak and improve opt-out for split disconnection logic.

There are now two ways to opt out from this feature:
* Set `SPLIT_MAX_CONNECTION_ERRORS` to 0. This will completely disable
  the connection status checks (also affects the slave matrix reset logic in
  matrix.c, though).
* Set `SPLIT_CONNECTION_CHECK_TIMEOUT` to 0. This will only disable the
  communication throttling while disconnected. Will make the firmware
  smaller.

* Make split disconnection logic work with custom transports.

Includes a fallback implementation for keyboards using a custom
split_util.c but not a custom matrix.c (currently no such keyboard seems
to be merged, though).

* Remove unnecessary include of timer.h

Co-authored-by: Joel Challis <git@zvecr.com>

Co-authored-by: Joel Challis <git@zvecr.com>

2 files changed, 19 insertions, 3 deletions

diff --git a/docs/feature_split_keyboard.md b/docs/feature_split_keyboard.md
index a84d384601..27df46a82a 100644
--- a/docs/feature_split_keyboard.md
+++ b/docs/feature_split_keyboard.md
@@ -204,6 +204,22 @@ If you're having issues with serial communication, you can change this value, as
 This sets the maximum number of milliseconds before forcing a synchronization of data from master to slave. Under normal circumstances this sync occurs whenever the data _changes_, for safety a data transfer occurs after this number of milliseconds if no change has been detected since the last sync. 
 
 ```c
+#define SPLIT_MAX_CONNECTION_ERRORS 10
+```
+This sets the maximum number of failed communication attempts (one per scan cycle) from the master part before it assumes that no slave part is connected. This makes it possible to use a master part without the slave part connected.
+
+Set to 0 to disable the disconnection check altogether.
+
+```c
+#define SPLIT_CONNECTION_CHECK_TIMEOUT 500
+```
+How long (in milliseconds) the master part should block all connection attempts to the slave after the communication has been flagged as disconnected (see `SPLIT_MAX_CONNECTION_ERRORS` above).
+
+One communication attempt will be allowed everytime this amount of time has passed since the last attempt. If that attempt succeeds, the communication is seen as working again.
+
+Set to 0 to disable this throttling of communications while disconnected. This can save you a couple of bytes of firmware size.
+
+```c
 #define SPLIT_TRANSPORT_MIRROR
 ```
 
diff --git a/docs/serial_driver.md b/docs/serial_driver.md
index ed989b0a15..3e89deffad 100644
--- a/docs/serial_driver.md
+++ b/docs/serial_driver.md
@@ -63,7 +63,7 @@ Configure the hardware via your config.h:
                                    //  5: about 19200 baud
 #define SERIAL_USART_DRIVER SD1    // USART driver of TX pin. default: SD1
 #define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
-#define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100
+#define SERIAL_USART_TIMEOUT 20    // USART driver timeout. default 20
 ```
 
 You must also enable the ChibiOS `SERIAL` feature:
@@ -105,10 +105,10 @@ Next configure the hardware via your config.h:
                                    //  3: 57600 baud
                                    //  4: 38400 baud
                                    //  5: 19200 baud
-#define SERIAL_USART_DRIVER SD1 // USART driver of TX and RX pin. default: SD1
+#define SERIAL_USART_DRIVER SD1    // USART driver of TX and RX pin. default: SD1
 #define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
 #define SERIAL_USART_RX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
-#define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100
+#define SERIAL_USART_TIMEOUT 20    // USART driver timeout. default 20
 ```
 
 You must also enable the ChibiOS `SERIAL` feature: