Double buffer documentation

[dzarda]

It would be interesting to have a demo or docs for the "not real" doublebuf functionality. Currently I have no idea how to use it - it didn't work when I simply "tried switching it on"

[dmitrystu]

The double-buffered feature can be applied to the bulk endpoint only. In this mode, the endpoint will not be NAKed after receive and will use a secondary buffer when core reads received data from the primary buffer.
There is no any special double-buffer mode for the OTGFS. This feature is implemented 'by design'. This option just doubles TX FIFO for the bulk endpoint.
With DEVFS you can configure a bulk EP as double-buffered, but you can't use the same physical endpoint for both IN and OUT. This is the limitation of the DEVFS and driver for the doublebuffered bulk or isochronous endpoints. Therefore the EP setup in the demo code is illegal for use with double-buffered bulk transfer.

// OK for the doublebuffered
#define CDC_EP0_SIZE    0x08
#define CDC_RXD_EP      0x01
#define CDC_TXD_EP      0x82
#define CDC_DATA_SZ     0x40
#define CDC_NTF_EP      0x83
#define CDC_NTF_SZ      0x08
#define HID_RIN_EP      0x84
#define HID_RIN_SZ      0x10

PS. I see no difference in the serial speed test for both configurations (single-buffered and double-buffered). Need to check transfers on the bus with the logic analyzer.

[homewsn]

Why do you automatically double the buffer for the isochronous endpoint? It seems to me that a separate USB_EPTYPE_DBLBUF option for an isochronous endpoint would be appropriate.

[dmitrystu]

According to chapter 32.5.4 "RM0367 Reference manual Ultra-low-power STM32L0x3 advanced ARM ® -based 32-bit MCUs" (same for the other DEVFS) isochronous endpoints are always doublebufered. For the OTGFS based devices, I kept this for behavioral compatibility.

[homewsn]

Thanks for the clarification.

[manuelbl]

@dmitrystu: Regarding the "not real" double buffered functionality... Is this a limitation of the OTGFS USB core? Is it because it is not possible to put additional data into the TX FIFO while the endpoint is transmitting?

[dmitrystu]

No, this is a limitation of the DEVFS core.