Espruino on P8 smartwatch - ST7789 display driver in Inline C

Having thought about it, I think the current implementation is probably the best you can do as for the reasons below, it is difficult/impossible to use double buffering. In addition, I think enabling and disabling the HW SPI is quite legitimate - it already happens to switch from DMA SPI to normal SPI to read single bytes. For some graphics screens, you have to do something similar to adjust to lower the bus speed for the touch controller if shares the display bus.

Double Buffering

My attempt at this - which works well when not using SPI flash - failed with a SPI timeout for - I think - the following reason. The driver starts a DMA operation to flush a buffer full of pixels and then returns to drawImage which accesses SPI flash to read the color palette. This read stops the DMA transfer with no completion interrupt so when it comes to try and flush the next buffer you get the SPI timeout.

I do not think that there is a general solution- other than synchronous single buffering - to stopping the CPU accessing the SPI flash in the time available during the DMA transfer. I tried blocking the SPI flash read while the DMA is in operation but this causes a sort of deadlock - #drawImage cannot fill the next buffer because it is waiting for a spi flash read which cannot complete because it is waiting for the bus to be released.

I have persevered with the driver as I would like to run different apps with different graphics requirements - you can see the result in the video below.

https://youtu.be/cHj_lXSNEv0

@fanoush - Many thanks for your invaluable help with this - looking forward to softdevice+bootloader upgrade package to move to SDK12 as I need the secure connection to connect my phone.