PI zero vs GD32F103 DMA SPI Transfers

I would have never have guessed that a 106Mhz microcontroller can run circles around 1.xGhz ARM11 using SPI DMA transfers of 256 bytes.

So, what I'm I talking about?

• GD32F103x is running at 106Mhz and using DMA for SPI and is configured as a slave.
• PI Zero is a Master (maybe is using DMA transfers) SPI.  
• SPI rate is 12Mhz (Close)

So, here is a logic analyzer trace:

Channel 4 is the GD32F103 raising the pin high at the top of the loop and lowering the pin after the DMA is configured waiting on the PI.

Using top to monitor CPU usage on the PI Zero:  spiExample, is taking 41% cpu usage (ouch).

Mem: 26956K used, 381340K free, 52K shrd, 1336K buff, 8332K cached
CPU:   0% usr  81% sys   0% nic  15% idle   0% io   0% irq   1% sirq
Load average: 1.41 0.80 0.61 1/97 247
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
  244   202 root     R     3412   1%  41% ./spiExample
   71     2 root     SW       0   0%  10% [irq/56-dwc_otg]
   73     2 root     SW       0   0%   8% [irq/56-dwc_otg_]
  130     2 root     SW       0   0%   7% [spi0]
  127     2 root     SW       0   0%   6% [irq/44-DMA IRQ]
  128     2 root     SW       0   0%   5% [irq/45-DMA IRQ]
   72     2 root     SW       0   0%   5% [irq/56-dwc_otg_]
  247   202 root     R     2476   1%   1% top
    3     2 root     SW       0   0%   1% [ksoftirqd/0]
    4     2 root     SW       0   0%   0% [ktimersoftd/0]
   84     2 root     SW       0   0%   0% [irq/81-uart-pl0]
  133     1 root     S    20892   5%   0% /usr/sbin/vcfiled
  148     1 root     S     4144   1%   0% wpa_supplicant -B -i wlan0 -c /etc/wpa
  202     1 root     S     2476   1%   0% -sh
  203     1 root     S     2348   1%   0% /sbin/getty -L tty1 0 vt100
    1     0 root     S     2344   1%   0% init
   99     1 root     S     2344   1%   0% /sbin/syslogd -n
  101     1 root     S     2344   1%   0% /sbin/klogd -n
  201     1 root     S     2344   1%   0% udhcpc -i wlan0 -t 10 -b
  124     1 root     S     2208   1%   0% /usr/sbin/dropbear -R

Need to look into this:

• Is the Pi Zero using interrupts for SPI (according to /proc/interrupts) no, it is only using DMA
• Is it PREMPT_RT kernel? 
• Doing some more testing it is setting a time between two back to back SPI packets.  But, it only lowers the CPU to 18% on the PI Zero.
• The GDF32F103 only takes 62us to copy memory for slave and reload DMA buffers. 

Well, next is to connect GPS, PWM (oneshot and decode) on the GD32F103.

Review, the GD32F103 is a I/O processor handling the following:

• PWM generation for the motors
• PWM/PPM decode RC transmitter
• I2C for LCD (Want to be able see status codes)
• (1) LED on the GD32F103 board for general status. 
• RS232 GPS
• SPI slave device to the PI Zero 
• The poll rate will be about 500hz
• PI zero is running BaseFlight/HackFlight.   

Anyway,  so, this week will be processing GPS,