Before this commit the USB VCP TX ring-buffer used the basic implementation
where it can only be filled to a maximum of buffer size-1. For a 1024 size
buffer this means the largest packet that can be sent is 1023. Once a
packet of this size is sent the next byte copied in goes to the final byte
in the buffer, so must be sent as a 1 byte packet before the read pointer
can be wrapped around to the beginning. So in large streaming transfers,
watching the USB sniffer you basically get alternating 1023 byte packets
then 1 byte packets.
This commit changes the ring-buffer implementation to a scheme that doesn't
have the full-size limitation, and the USB VCP driver can now achieve a
constant stream of full-sized packets. This scheme introduces a
restriction on the size of the buffer: it must be a power of 2, and the
maximum size is half of the size of the index (in this case the index is
16-bit, so the maximum size would be 32767 bytes rounded to 16384 for a
power-of-2). But this is not a big limitation because the size of the
ring-buffer prior to this commit was restricted to powers of 2 because it
was using a mask-based method to wrap the indices.
For an explanation of the new scheme see
https://www.snellman.net/blog/archive/2016-12-13-ring-buffers/
The RX buffer could likely do with a similar change, though as it's not
read from in chunks like the TX buffer it doesn't present the same issue,
all that's lost is one byte capacity of the buffer.
USB VCP TX throughput is improved by this change, potentially doubling the
speed in certain cases.
Enabled by default, but disabled when REPL is connected to the VCP (this is
the existing behaviour). Can be configured at run-time with, eg:
pyb.USB_VCP().init(flow=pyb.USB_VCP.RTS | pyb.USB_VCP.CTS)
Prior to this commit the USB CDC used the USB start-of-frame (SOF) IRQ to
regularly check if buffered data needed to be sent out to the USB host.
This wasted resources (CPU, power) if no data needed to be sent.
This commit changes how the USB CDC transmits buffered data:
- When new data is first available to send the data is queued immediately
on the USB IN endpoint, ready to be sent as soon as possible.
- Subsequent additions to the buffer (via usbd_cdc_try_tx()) will wait.
- When the low-level USB driver has finished sending out the data queued
in the USB IN endpoint it calls usbd_cdc_tx_ready() which immediately
queues any outstanding data, waiting for the next IN frame.
The benefits on this new approach are:
- SOF IRQ does not need to run continuously so device has a better chance
to sleep for longer, and be more responsive to other IRQs.
- Because SOF IRQ is off, current consumption is reduced by a small amount,
roughly 200uA when USB is connected (measured on PYBv1.0).
- CDC tx throughput (USB IN) on PYBv1.0 is about 2.3 faster (USB OUT is
unchanged).
- When USB is connected, Python code that is executing is slightly faster
because SOF IRQ no longer interrupts continuously.
- On F733 with USB HS, CDC tx throughput is about the same as prior to this
commit.
- On F733 with USB HS, Python code is about 5% faster because of no SOF.
As part of this refactor, the serial port should no longer echo initial
characters when the serial port is first opened (this only used to happen
rarely on USB FS, but on USB HS is was more evident).
This patch adds the configuration MICROPY_HW_USB_ENABLE_CDC2 which enables
a new USB device configuration at runtime: VCP+VCP+MSC. It will give two
independent VCP interfaces available via pyb.USB_VCP(0) and pyb.USB_VCP(1).
The first one is the usual one and has the REPL on it. The second one is
available for general use.
This configuration is disabled by default because if the mode is not used
then it takes up about 2200 bytes of RAM. Also, F4 MCUs can't support this
mode on their USB FS peripheral (eg PYBv1.x) because they don't have enough
endpoints. The USB HS peripheral of an F4 supports it, as well as both the
USB FS and USB HS peripherals of F7 MCUs.
This patch adds support in the USBD configuration and CDC-MSC-HID class for
high-speed USB mode. To enable it the board configuration must define
USE_USB_HS, and either not define USE_USB_HS_IN_FS, or be an STM32F723 or
STM32F733 MCU which have a built-in HS PHY. High-speed mode is then
selected dynamically by passing "high_speed=True" to the pyb.usb_mode()
function, otherwise it defaults to full-speed mode.
This patch has been tested on an STM32F733.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
