66 lines
3.8 KiB
ReStructuredText
66 lines
3.8 KiB
ReStructuredText
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SoundWire Error Handling
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The SoundWire PHY was designed with care and errors on the bus are going to
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be very unlikely, and if they happen it should be limited to single bit
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errors. Examples of this design can be found in the synchronization
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mechanism (sync loss after two errors) and short CRCs used for the Bulk
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Register Access.
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The errors can be detected with multiple mechanisms:
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1. Bus clash or parity errors: This mechanism relies on low-level detectors
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that are independent of the payload and usages, and they cover both control
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and audio data. The current implementation only logs such errors.
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Improvements could be invalidating an entire programming sequence and
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restarting from a known position. In the case of such errors outside of a
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control/command sequence, there is no concealment or recovery for audio
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data enabled by the SoundWire protocol, the location of the error will also
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impact its audibility (most-significant bits will be more impacted in PCM),
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and after a number of such errors are detected the bus might be reset. Note
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that bus clashes due to programming errors (two streams using the same bit
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slots) or electrical issues during the transmit/receive transition cannot
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be distinguished, although a recurring bus clash when audio is enabled is a
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indication of a bus allocation issue. The interrupt mechanism can also help
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identify Slaves which detected a Bus Clash or a Parity Error, but they may
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not be responsible for the errors so resetting them individually is not a
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viable recovery strategy.
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2. Command status: Each command is associated with a status, which only
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covers transmission of the data between devices. The ACK status indicates
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that the command was received and will be executed by the end of the
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current frame. A NAK indicates that the command was in error and will not
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be applied. In case of a bad programming (command sent to non-existent
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Slave or to a non-implemented register) or electrical issue, no response
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signals the command was ignored. Some Master implementations allow for a
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command to be retransmitted several times. If the retransmission fails,
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backtracking and restarting the entire programming sequence might be a
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solution. Alternatively some implementations might directly issue a bus
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reset and re-enumerate all devices.
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3. Timeouts: In a number of cases such as ChannelPrepare or
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ClockStopPrepare, the bus driver is supposed to poll a register field until
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it transitions to a NotFinished value of zero. The MIPI SoundWire spec 1.1
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does not define timeouts but the MIPI SoundWire DisCo document adds
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recommendation on timeouts. If such configurations do not complete, the
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driver will return a -ETIMEOUT. Such timeouts are symptoms of a faulty
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Slave device and are likely impossible to recover from.
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Errors during global reconfiguration sequences are extremely difficult to
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handle:
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1. BankSwitch: An error during the last command issuing a BankSwitch is
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difficult to backtrack from. Retransmitting the Bank Switch command may be
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possible in a single segment setup, but this can lead to synchronization
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problems when enabling multiple bus segments (a command with side effects
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such as frame reconfiguration would be handled at different times). A global
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hard-reset might be the best solution.
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Note that SoundWire does not provide a mechanism to detect illegal values
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written in valid registers. In a number of cases the standard even mentions
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that the Slave might behave in implementation-defined ways. The bus
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implementation does not provide a recovery mechanism for such errors, Slave
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or Master driver implementers are responsible for writing valid values in
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valid registers and implement additional range checking if needed.
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