So apparently the STM32F401's SPI DMA is even more buggy than the 405's.
Worked around an intermittent stall/timeout by busy-waiting Upstream's
packet length transmission and reception, instead of DMA-ing it like the
packet body. Ugh...
correctly now, either because SPI is in 16-bit mode, or because I found
all the other bugs!
Doubled SPI baudrate to 10.5Mbps. Transfer speed now limited (again) by
Downstream's lack of FIFO buffering in the USB host controller.
Also disabled DMA transaction half-complete interrupt in
stm32f4xx_hal_dma.c, as it wasn't doing anything useful.
SPI peripheral library. At this speed SPI requires ~60% CPU time at -Og
optimisation level. This could be further improved by trimming down the
SPI interrupt. But...
Speed is now limited by Downstream's single-packet-per-URB restriction,
to about 460kB/s. USB Middleware does not implement TX FIFO empty
interrupt, so a bit of work is required here.
Although I am not entirely convinced this is necessary, as the SPI data
stall issue only appeared with optimisation off (-O0). Perhaps re-visit
this if Upstream needs more free CPU time later...
Robert Fisk