midi host: raise default RX FIFO above EP size, document drain requirement

Follow-up to #3239. tuh_midi_stream_read terminates on cable-number
transitions, leaving residue in the FIFO. With the default RX FIFO
sized equal to one bulk packet, the next bulk IN transfer fails to
queue and the driver silently stops receiving. Raising the default
to 2x bulk gives single-call apps a full packet of headroom and
documents the drain-loop expectation.

Reproduced with Akai LPD8 mk1 (VID 09E8 PID 0075) on STM32H753 DWC2
host; fixed with this patch. See #3613 for full repro + captures.

1 files changed, 14 insertions, 2 deletions

diff --git a/src/class/midi/midi_host.h b/src/class/midi/midi_host.h
index b9ab0130d..4eefed4ba 100644
--- a/src/class/midi/midi_host.h
+++ b/src/class/midi/midi_host.h
@@ -38,11 +38,16 @@ extern "C" {
 // Class Driver Configuration
 //--------------------------------------------------------------------+
 #ifndef CFG_TUH_MIDI_RX_BUFSIZE
-  #define CFG_TUH_MIDI_RX_BUFSIZE TUH_EPSIZE_BULK_MAX
+  // Default sized to 2x the bulk endpoint to absorb residue left in the FIFO
+  // when tuh_midi_stream_read() stops early on a cable-number transition.
+  // Sizing this equal to the endpoint packet size (the historical default)
+  // can cause the next bulk IN transfer to fail to queue silently, wedging
+  // the stream. See the drain-loop note on tuh_midi_stream_read() below.
+  #define CFG_TUH_MIDI_RX_BUFSIZE (2 * TUH_EPSIZE_BULK_MAX)
 #endif
 
 #ifndef CFG_TUH_MIDI_TX_BUFSIZE
-  #define CFG_TUH_MIDI_TX_BUFSIZE TUH_EPSIZE_BULK_MAX
+  #define CFG_TUH_MIDI_TX_BUFSIZE (2 * TUH_EPSIZE_BULK_MAX)
 #endif
 
 #ifndef CFG_TUH_MIDI_EP_BUFSIZE
@@ -150,6 +155,13 @@ uint32_t tuh_midi_stream_write(uint8_t idx, uint8_t cable_num, const uint8_t *p_
 // Note that this function ignores the CIN field of the MIDI packet
 // because a number of commercial devices out there do not encode
 // it properly.
+//
+// NOTE: this function terminates when it encounters an event whose cable
+// number differs from the one being returned. Applications should invoke
+// it in a loop until it returns 0 (or until tuh_midi_read_available()
+// returns 0) to guarantee the stream FIFO is fully drained per callback.
+// Leaving bytes in the FIFO across callbacks can prevent subsequent bulk
+// IN transfers from landing.
 uint32_t tuh_midi_stream_read(uint8_t idx, uint8_t *p_cable_num, uint8_t *p_buffer, uint16_t bufsize);
 
 #endif