Faster hardware SPI writes on SAMD

[monroewilliams]

In the Adafruit_SPIDevice class, when running on the SAMD architecture in the hardware SPI case, use the SPIClass::transfer(txbuf, rxbuf, count, block) method when writing instead of looping over individual bytes.

This makes use of DMA and is noticeably faster.

[monroewilliams]

Hmm... I see the automated build is failing, and it looks like the transfer function I'm calling doesn't exist?
I'm building this under PlatformIO using the current default version of the atmelsam core (looks like 7.0.0 according to their versioning), and not seeing this failure locally.

[ladyada]

################################################################################ SWITCHING TO arduino:samd:arduino_zero_native Installing arduino:samd ✓ arduino:samd 1.8.12 1.8.12 Arduino SAMD Boards (32-bits ARM Cortex-M0+) ################################################################################ i2c_address_detect.ino ❌

arduino samd zero (non-adafruit samd) is not compiling

[monroewilliams]

Gotcha. That's this core, right? https://github.com/arduino/ArduinoCore-samd

There doesn't appear to be an analogue to void transfer(const void* txbuf, void* rxbuf, size_t count, bool block); in that core, so I guess I'll just need to disable this optimization for that case.

I don't see a preprocessor define I can use to specifically check for the presence of that method. I assume the check I'll need to make is that we're using the Adafruit core.

What's the most appropriate #if to use for that? Is there some sort of versioning #define that can tell cleanly which core is being used?

It looks like I could differentiate that it's using the Adafruit version of SPI.h with an ugly hack like #if defined(_ADAFRUIT_ZERODMA_H_), but that just seems wrong.

[ladyada]

yeah i think you will have to 'fake' it with an #ifdef and just comment why - we added some rough DMA support but we aren't caught up on the arduino core so we weren't able to PR the change!

[ladyada]

thanks! will try it