uefi: serial: add read_exact() and write_exact() + fix core::fmt::Write for Serial#1875
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Initial comments in. For ease of review, I'd like to move the read/write changes to a separate PR, those seem independent of the new methods being added.
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thanks for your helpful remarks and ideas! The new state is much better: no infinite loops, cleaner interfaces. I tested this on real hardware - multiple times. Hope we are good to go now - looking forward to your feedback! |
Knowing how long a single byte takes to transmit at the current baud rate lets retry logic stall for just the right amount of time - long enough for the FIFO to drain or fill, short enough to stay responsive. Required groundwork for the exact-read/write helpers that follow.
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Callers that simply want to send or receive a fixed number of bytes should not have to implement their own retry loop. These helpers absorb the TIMEOUT-and-retry pattern internally, keeping call sites clean. Endless-loop protection ensures a hardware fault cannot stall the system forever. Validated on real hardware with reading/writing 16 KiB chunks.
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On real hardware, Status::TIMEOUT is common even during normal operation, not just on failure. The previous implementation treated it as a hard error, silently dropping bytes. Retrying until all bytes are sent makes the fmt::Write impl reliable and match the expectations of its callers. This uses an the same endless loop protection mentioned earlier.
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Serial Protocol Improvements
Serial::read_exact()andSerial::write_exact()methods, which block until the entire buffer is read or written, automatically retrying on timeouts and stalling based on baud rate to avoid busy-waiting. These methods help prevent partial I/O and improve reliability when communicating over serial devices.fmt::Writeimplementation forSerialto usewrite_exact()Timing and Reliability Enhancements
duration_per_byte_estimateandduration_fifo_estimate) to conservatively estimate the time needed for serial operations, taking into account baud rate, data bits, parity, and stop bits. These are used to determine appropriate stalling durations between retries.Testing and Documentation