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Code Issues Releases Activity

usb cdc: avoid need for ZLP by using tx buffers smaller than max

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USB CDC uses BULK endpoints to send/receive data.  Typically,
a USB host will enqueue large buffers on its IN (device-to-host)
URBs.  These buffers are larger than the max packet size for the
bulk endpoint.

The USB standard requires that an IN transfer ends when one of
these is true:
 * a short packet (ie. less than max packet size) is sent by the
   device
 * a zero length packet (ZLP)
 * enough packets that the entire host buffer is filled

Our device implementation never sends ZLPs.  We sometimes send
packets that are exactly max-packet-size bytes long.  This would
result in partially filling a host buffer without signalling (via
ZLP) that the transmission had finished.  The host would then wait
until the next transfer had taken place before processing the first
data, thus delaying the first data.

This change simply forces all of our transfers to be short packets
and avoids the need to worry about zero length packets.  This is
at the cost of some efficiency on the host side since its large
buffers will only ever be partially filled.

Conflicts:
	flight/PiOS/STM32F30x/pios_usb_cdc.c
This commit is contained in:
Stacey Sheldon 2013-03-16 23:30:18 -04:00 committed by Alessio Morale
parent 366c573bab
commit c22daec1b0
2 changed files with 12 additions and 2 deletions
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7
flight/pios/stm32f10x/pios_usb_cdc.c
View File

@ -69,7 +69,12 @@ struct pios_usb_cdc_dev {
uint32_t tx_out_context;
uint8_t rx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH];
uint8_t tx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH];
/*
* NOTE: This is -1 as somewhat of a hack. It ensures that we always send packets
* that are strictly < maxPacketSize for this interface which means we never have
* to bother with zero length packets (ZLP).
*/
uint8_t tx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH - 1];
uint32_t rx_dropped;
uint32_t rx_oversize;

7
flight/pios/stm32f4xx/pios_usb_cdc.c
View File

@ -73,7 +73,12 @@ struct pios_usb_cdc_dev {
uint8_t rx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH] __attribute__ ((aligned(4)));
volatile bool rx_active;
uint8_t tx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH] __attribute__ ((aligned(4)));
/*
* NOTE: This is -1 as somewhat of a hack. It ensures that we always send packets
* that are strictly < maxPacketSize for this interface which means we never have
* to bother with zero length packets (ZLP).
*/
uint8_t tx_packet_buffer[PIOS_USB_BOARD_CDC_DATA_LENGTH - 1] __attribute__ ((aligned(4)));
volatile bool tx_active;
uint8_t ctrl_tx_packet_buffer[PIOS_USB_BOARD_CDC_MGMT_LENGTH] __attribute__ ((aligned(4)));