Re: [Libusb-devel] Slow USB bulk transfer

[mohammed a. <abu...@gm...>]

Alright then, thanks for the explanation. I was using 512 because I thought
I had to abide by the maxPacketsize of the endpoint I am using.

Now the second problem is that I get a zero as a return value from
usb_bulk_write. It is still the same code but with changing the size value
of usb_bulk_write to 1 MB instead of 512.

Looking at linux.c, this means that zero bytesdone have been returned and I
wrote nothing to the device..!!

What could be going wrong. As you mentioned, bulk is synchronous, so it had
to wait until something is written and then come back..I also have the
timeout value as zero...which according to my understanding is ignored
altogether if we doing bulk read/write...



On 2/28/07, Tim Roberts <ti...@pr...> wrote:
>
> mohammed abuhijle wrote:
> >
> > I have the following code that takes more than a min to finisg:
>
> That's right.
>
>
> > /*write 40 Mbytes worth of data to the bulk endpoint*/
> > /*i am calculating this by: 512 (packet size) * 2 = 1 KB, * 1000 =
> > 1MB, * 40 = 40 MB*/
> > for (i = 0; i < 2 * 1000 * 40 ; i++){
> > ret = usb_bulk_write(cypress_dev_handle, epi,bytes, 512, 0 /*the
> > timrout is ignored for bulk transfer*/);
> > //printf ("i is:%d\t ret is %d\t bytes is %s\n",i, ret,bytes);
> > }
>
> Remember that USB is a "scheduled" bus.  The USB host controller has to
> have a frame all planned out before the frame begins.  In your case, you
> are sending exactly one packet down, which then gets scheduled into a
> frame, and because usb_bulk_write is synchronous, you then wait until
> the transfer is complete, thereby wasting the rest of the frame.  That
> means you cannot do any better than one packet per frame, 512k bytes per
> second, which would make 40MB take 80 seconds.
>
> The USB subsystem will happily chop up a large transfer into
> packet-sized pieces as needed, and when it does so, it will pack in as
> many packets as it can into a single frame.  You can send megabytes to
> usb_bulk_write, and get your 40MB transferred in a second or two.
>
> --
> Tim Roberts, ti...@pr...
> Providenza & Boekelheide, Inc.
>
>
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