[jedgarpark]

Fixes #11692.

tuh_max3421_spi_cs_api() can't wait for an SPI bus that other code holds, so it skips the transfer silently. TinyUSB then acts on register values it never read. When the dropped write is the one that starts a transfer, busy_lock stays set with no transfer-done interrupt to clear it, and the host stalls until reset. Python code, a display refresh or an SD card read can all run background tasks while holding the bus, so tuh_task() can run at any of those times.

This adds max3421e_spi_busy(), which try-locks the MAX3421E's bus and releases it again. usb_background() checks it before tuh_task(), and when the bus is held it reschedules itself instead, so the host task only runs when its transfers can go through. Boards without CIRCUITPY_MAX3421E are unchanged.

Testing:

• @mikeysklar's debugger runs on a Feather RP2350 under a 4 MHz SPI write load: unpatched, stuck 9/15 with 1 to 3 dropped transfers in each stuck run; patched, stuck 0/10 with no dropped transfers (details in the issue).
• Feather RP2350 + MAX3421E FeatherWing sharing board.SPI() with an HX8357 TFT and an SD card, enumerating and flashing a USB MIDI device over many runs.

🤖 Generated with Claude Code

[mikeysklar]

I moved some hardware around to free up a Feather doubler and I'm running some tests just to be safe. Will get back to you this afternoon.

13/34 trials, about 27 min 00 s left
Estimated finish 14:13

Build  Test       Done  Stuck  Drops  Errs  HF
Parent Host alone  2/2      0      0     0   0
Parent Real SD    10/10    10     66     0   0
Parent SPI load    1/5      0      0     0   0
PR     Host alone  0/2      0      -     0   0
PR     Real SD     0/10     0      -     0   0
PR     SPI load    0/5      0      -     0   0

[mikeysklar]

Tested this PR and its parent on a Feather RP2350 with real SD reads.

Build	Test	Stuck until reset
Parent	Host alone	0/2
Parent	Real SD	10/10
Parent	SPI load	2/5
PR	Host alone	0/2
PR	Real SD	0/10
PR	SPI load	0/5

No hard fault. Not tested with SD-over-USB on, or on other ports.