FYI about DMA issues on esp32

[amirgon]

@tve

I modified the python xpt2046 driver to perform a “read” (get X Y coords of a click) using a single SPI transaction and using the “16 cycle overlapped” mode.

Hi @tve!
Could you explain the motivation for this change? What was the problem you were trying to solve?

@tve

After discovering all this in test cases I switched the display driver to run full-duplex transactions (it’s just a matter of commenting out the "flags": esp.SPI_DEVICE.HALFDUPLEX line) and now everything works. Argh!

It might be worth making this change in the display driver so others don’t hit similar bugs… I don’t see any downside.

There is a good reason it’s configured as half-duplex.
See: Can't mount SD card and display at the same time when sharing the same SPI bus · Issue #69 · lvgl/lv_binding_micropython · GitHub

In a nutshell, Full-duplex transactions are not compatible with the dummy bit workaround and limit SPI speed to 26MHz.
On the other hand, half-duplex with the dummy bit workaround allow speeds up to 80MHz. I usually use the display driver at 40MHz.

It’s possible, however, to add another parameter to ili9341 constructor to configure it as full-duplex when needed, if it helps to solve some problems and when the SPI speed is less important.
If you want to help, we always welcome PRs!