qcwdfserial: fix latency small chunk reads regression after QUD-1837 ReadIntervalTimeout

[Cristian Manca (CristianManca)]

Summary

This PR fixes a latency regression introduced by commit
9e60a01974f7bd802c5e6660c6c12812e4d819fc
(fix(wdfserial): implement ReadIntervalTimeout inter-byte gap semantics [QUD-1837]).

The regression is most visible with small read sizes (especially 1-byte reads): data already present in the ring buffer could still follow the timer-based path, adding unnecessary delay and reducing throughput.

Root cause

With ReadIntervalTimeout enabled, the read path could wait on the inter-byte timer even when bytes were already buffered, effectively serializing delivery and increasing per-read latency.

Code-level changes

In src/windows/wdfserial/QCRD.c, inside QCRD_ReadRequestHandlerThread (READ_THREAD_REQUEST_ARRIVE_EVENT path):

• The RI path is now gated with:
  • if (useReadInterval && QCUTIL_RingBufferBytesUsed(rxBuffer) == 0)
• If data is already buffered, the code now follows immediate-drain logic instead of waiting for timeout.
• Added explicit guard to preserve QUD-1837 semantics when a request is already pending in TimeoutReadQueue and ReadQueue is empty:
  • keep waiting for REQUEST_TIMEOUT_EVENT.
• Before immediate drain in RI mode, pending timer state is cleared:
  • KeCancelTimer(&pDevContext->ReadTimer);
  • KeClearEvent(&pDevContext->ReadRequestTimeoutEvent);
• During RI bypass, requests are served from ReadQueue (not drained from TimeoutReadQueue) so timeout-queued requests remain timer-governed.

Impact

• Significant latency reduction for small-buffer reads (1 byte and few-byte reads).
• Throughput recovery for frequent small-chunk read patterns.

[hangzqcom]

Update comment to

// If useReadInterval is true but data is already buffered,
// cancel any pending inter-byte timer before draining.
// KeCancelTimer returns FALSE if the DPC is already queued;
// in that case ReadTimeoutDpc will fire after KeClearEvent and
// set ReadRequestTimeoutEvent again. READ_THREAD_REQUEST_TIMEOUT_EVENT
// will then find the ring buffer empty (drained below) and complete
// any pending TimeoutReadQueue request with STATUS_TIMEOUT / 0 bytes.
// This is correct RI semantics: the inter-byte gap elapsed and no
// data was available for that request (it was consumed by ReadQueue).

[Cristian Manca (CristianManca)]

Comment updated

[hangzqcom]

Fix else block indentation

[Cristian Manca (CristianManca)]

Indentation fixed