linux-next/Documentation/driver-api/soundwire/error_handling.rst

========================
SoundWire Error Handling
========================

The SoundWire PHY was designed with care and errors on the bus are going to
be very unlikely, and if they happen it should be limited to single bit
errors. Examples of this design can be found in the synchronization
mechanism (sync loss after two errors) and short CRCs used for the Bulk
Register Access.

The errors can be detected with multiple mechanisms:

1. Bus clash or parity errors: This mechanism relies on low-level detectors
   that are independent of the payload and usages, and they cover both control
   and audio data. The current implementation only logs such errors.
   Improvements could be invalidating an entire programming sequence and
   restarting from a known position. In the case of such errors outside of a
   control/command sequence, there is no concealment or recovery for audio
   data enabled by the SoundWire protocol, the location of the error will also
   impact its audibility (most-significant bits will be more impacted in PCM),
   and after a number of such errors are detected the bus might be reset. Note
   that bus clashes due to programming errors (two streams using the same bit
   slots) or electrical issues during the transmit/receive transition cannot
   be distinguished, although a recurring bus clash when audio is enabled is a
   indication of a bus allocation issue. The interrupt mechanism can also help
   identify Slaves which detected a Bus Clash or a Parity Error, but they may
   not be responsible for the errors so resetting them individually is not a
   viable recovery strategy.

2. Command status: Each command is associated with a status, which only
   covers transmission of the data between devices. The ACK status indicates
   that the command was received and will be executed by the end of the
   current frame. A NAK indicates that the command was in error and will not
   be applied. In case of a bad programming (command sent to non-existent
   Slave or to a non-implemented register) or electrical issue, no response
   signals the command was ignored. Some Master implementations allow for a
   command to be retransmitted several times.  If the retransmission fails,
   backtracking and restarting the entire programming sequence might be a
   solution. Alternatively some implementations might directly issue a bus
   reset and re-enumerate all devices.

3. Timeouts: In a number of cases such as ChannelPrepare or
   ClockStopPrepare, the bus driver is supposed to poll a register field until
   it transitions to a NotFinished value of zero. The MIPI SoundWire spec 1.1
   does not define timeouts but the MIPI SoundWire DisCo document adds
   recommendation on timeouts. If such configurations do not complete, the
   driver will return a -ETIMEOUT. Such timeouts are symptoms of a faulty
   Slave device and are likely impossible to recover from.

Errors during global reconfiguration sequences are extremely difficult to
handle:

1. BankSwitch: An error during the last command issuing a BankSwitch is
   difficult to backtrack from. Retransmitting the Bank Switch command may be
   possible in a single segment setup, but this can lead to synchronization
   problems when enabling multiple bus segments (a command with side effects
   such as frame reconfiguration would be handled at different times). A global
   hard-reset might be the best solution.

Note that SoundWire does not provide a mechanism to detect illegal values
written in valid registers. In a number of cases the standard even mentions
that the Slave might behave in implementation-defined ways. The bus
implementation does not provide a recovery mechanism for such errors, Slave
or Master driver implementers are responsible for writing valid values in
valid registers and implement additional range checking if needed.