Re: [Ftdi-usb-sio-devel] FT245BM transfer rates

[Karl R. <ka...@ho...>]

Hi!

>I believe the bottleneck is how the transactions are sent to the usb stack. 
>One packet at a time. The larger the writes/reads you can do the better - 
>ie reduce the number of system calls.

Well, my system handles 500 kByte/s bulk transfers from my DigiCam with a 
very light system-load. (Celeron 450)

>But some driver hacking would be required to attempt sending a bigger 
>packet than 128bytes and let the usb stack split that packet up. However I 
>don't know if the 245 would be happy with this.

I'm not sure if its reltated to the maximum packed size -
the '245.pdf says:

384 Byte FIFO Tx buffer / 128 Byte FIFO Rx Buffer


>It would be an interesting experiment though and would should give a lot 
>more performance.

I've took a closer look on the handshake signals(PC to '245 transfer):
It looks like the '245 gets *only* one 64 byte packet every 1 ms(aka 1 kHz 
USB-clock) - 384 should be OK, IMHO.


>Perhaps another of the usb serial drivers has implemented a multiple buffer 
>scheme to improve performance but I haven't looked at them in AGES so don't 
>know.

Anybody knows which files are involved in a USB-bulk-transfer?
I've only found ftdi_sio.c, usbserial.c  so far :-(


Karl






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