I’ve kept working on my Raspberry Pi SPI driver and the last few updates should be quite interesting to people following this. Aside from fixing a silly bug a few days ago (I was releasing the chipselect lines when I shouldn’t have been), I’ve done some work to no longer require the switchPinCtrl tool and to implement IRQ mode.
IRQ mode means that the driver no longer uses 100% of the CPU when doing transfers, running a busy loop checking the flags to read/write the FIFO. Instead, it waits until the SPI hardware tells it that it needs to do something, and means other software can get on and do things at the same time. In testing SPI transfers to/from some AT45DB161D flash memory chips, I’m using less than 5% of the Raspberry Pi’s CPU including the top process.
I’m testing the SPI with those chips at 7.8 MHz (250 MHz ÷ 32) and it is working very reliably indeed, with no data errors reading and writing random data. At 15.6 MHz the process was quite unreliable, but I think that’s more to do with my jumble of jumper wires just not managing those sorts of speeds which is rather predictable. I have also tested with a DS3234 RTC chip which works very well.
I’ve also incorporated a small snippet of code to put the SPI pins in the correct ALT mode (ALT 0) so that the SPI hardware can drive them, instead of them being set up as GPIO pins. Previously I asked that folks run a command-line tool to do this but this was leading to a great deal of confusion (especially on my part!) so I put the code into my driver. This is not where the code belongs, really this should be in a pinmux/pinctrl driver, but it’ll do for now.
Update: Updated the link to the git branch, now points at the branch with both the I2C and SPI drivers in.
