atsamd5x: improve USBCDC

[sago35]

The following approaches were used to speed up the process.

• It used to send every 1 byte, but it now holds a maximum of 63 bytes before sending
• if the buffer is full, send buffer
• If there is data at the time of the handleUSBIRQ every 1ms, send buffer

Unlike #1452, it does not require additional memory.
The code is a little more complicated, but it looks like it works very well.

• 13 byte : fmt.Printf("hello world\r\n")
  • old : 202.814 us
  • new : 51.330 us
• 11 byte : println("hello world")
  • old : 171.274 us
  • new : 11.920 us
• 1024 byte + "\r\n"
  • old : 13.999962 ms
  • new : 1.150692 ms
• 1024 * 10 byte + "\r\n"
  • old : 139.649294 ms
  • new : 11.612682 ms

interrupt.Disable() cannot be built unless interp-rewrite is merged.

Samd2x can be made faster in the same way.
Once we have reviewed this PR, we will create a PR.

[sago35]

The reason why it is set to draft is that we need to merge the interp-rewrite branches to be able to build.
I would like a review.

I tested it with the following code with WioTerminal.

package main

import (
	"fmt"
	"machine"
	"time"
)

var buf = [1024 * 10]byte{}

var (
	debugPort1 = machine.BCM3
)

func main() {
	debugPort1.Configure(machine.PinConfig{Mode: machine.PinOutput})

	for i := range buf {
		if ((i + 1) & 0x0F) == 0 {
			buf[i] = byte(fmt.Sprintf("%X", byte(i>>4)&0xF)[0])
		} else {
			buf[i] = '.'
		}
	}
	buf[0] = '+'

	for {
		debugPort1.High()

		machine.UART0.Write(buf[:1024*10])
		//machine.UART0.Write(buf[:1024])
		machine.UART0.Write([]byte("\r\n"))
		//fmt.Printf("hello world\r\n")
		//println("hello world")

		debugPort1.Low()
		time.Sleep(time.Second)
	}
}

[sago35]

• Use udd_ep_in_cache_buffer[usb_CDC_ENDPOINT_IN][128] as two 64 byte buffers.
  • The term udd_ep_in_cache_buffer is abbreviated to buffer below
• WriteByte()
  • Set c byte to buffer[TxIdx].
  • Can write up to 63 bytes.
  • The reason it's 63 bytes instead of 64 bytes is because TxIdx & 0x3F treats it as a size
  • Call Flush() if you've written to the maximum
• Flush()
  • Flush() sends the data in the buffer
  • Calculate the size to be sent using TxIdx & 0x3F
  • When the size is greater than 0, it is actually sent to USBCDC
  • After sending, set TxIdx to the next buffer
    • For example, when TxIdx is 0 to 63, TxIdx is set to 64. And Set TxIdx to 0 when the value is 64 to 127.
  • Set UART0.sent to true if it is sent once.
    • If not using UART0.sent, USBCDC status is not known at WriteByte() time.
• handleUSBIRQ()
  • Interrupts every 1ms
  • Calling Flush()

[sago35]

It is buildable on the current dev branch so it is ready for review.

I wish I could make the implementation more straightforward, but I can't think of a good way to do it right now.

The code for testing can be found at
#1481 (comment)

I checked it on Windows 10 and found the following

• 1024 * 10 byte + "\r\n"
  • old : 514.359 ms
  • new : 11.633ms

[deadprogram]

The code does appear to work. I made a few comments about using const values for clarity.

@aykevl do you have any feedback?

[sago35]

Updated. @deadprogram

[deadprogram]

Made a few small comments @sago35 that look like we can shorten by a couple of lines of code.

[sago35]

Updated. @deadprogram

I thought USBCDC.Flush() should not be changed.
Should I change it?

[deadprogram]

I think this is now fine how it is. @sago35 can you please squash into a single commit? Thank you!

[sago35]

squashed. @deadprogram

[deadprogram]

This is really great, thank you for working on it @sago35 just waiting for CI to finish to merge.

Now we just need the same thing for SAMD21! 😸

[sago35]

I will prepare the samd21 version as well.