On Mon, 23 Apr 2007, nesta wrote:
> hi all,
> i have 2 questions:
>
> 1) i make simple while loop,at the host side, in which i m receiving the data
> using the read function of usb-skeleton module,it is as follows:
> ***************************************************************************
> do
> {
> insize =read (fd,(char*)inbuff, sizeof (inbuff) );
> printf("insize is %d\n",insize);
> printf(" %s\n",inbuff);
> }while (strcmp(inbuff,"end"));
> ****************************************************************************
> my aim is : when the gadget has something to send to the host this while loop
> will print what the hosts receives from the gadget.
>
> but this does not happen when i use the skel_read function,it is in
> usb-skeleton.c,as when the gadget does not have anything to send to the
> host,i
> found that the read function is non-blocking and it prints the following
> forever:
> insize is -1
> black line //i think this is because inbuff is empty
The read function _is_ blocking, but it times out after 10 seconds.
> anyway i found that there is a function in skel_read function, named by
> usb_bulk_msg and the last argument of it, is the timeout and if i set this
> timeout to zero it will stop printing insize is -1 and thus it becomes a
> blocking function.
Because you removed the timeout.
> so now if the gadget is not sending anything to the host, the while loop
> stops
> printing
> insize is -1
> bu when the gadget really sends a word like "hello host" for example, the
> host
> really receives it but still printing the phrase of insize is 64 i.e the
> output
> is as follows:
>
> insize is 64
> hello host
> insize is 64
> insize is 64 and it prints this phrase forever
> so why it just receives 64B while i was really sending 2KB buffer?
You are contradicting yourself. Above you said the gadget sends the words
"hello host", which is 10 bytes, but now you say the gadget sends a
2048-byte buffer. Which is it?
> actually what i want exactly is when the gadget sends "hello host" the while
> looop output is :
>
> insize is 2048 // 2KB=2048
> hello host
Why do you want your program to print that insize is 2048 when it received
only 10 bytes from the gadget?
Any funny behavior you encounter is probably caused by the gadget. If you
know what it is really sending, then you should also know why the host
behaves the way it does.
> and doesnt print anything else?
>
> 2) can i make the following steps in reading instead of usb_bulk_msg:
> 1.urb = usb_alloc_urb(0, GFP_KERNEL);
> 2.buf = usb_buffer_alloc(dev->udev, writesize, GFP_KERNEL,
> &urb->transfer_dma);
If buf is for reading, shouldn't you use readsize instead of writesize?
> 3.copy_to_user(buf, user_buffer, readsize
> 4.usb_fill_bulk_urb( urb, dev->udev,
> usb_rcvctrlpipe(dev->udev, dev->bulk_in_endpointAddr),
> buf, readsize, skel_read_bulk_callback, dev);
> 5.urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
> 6.usb_submit_urb
>
> the location of copy_to_user is correct?or it should be put after
> usb_submit_urb?
It definitely should go after usb_submit_urb. In fact, it shouldn't
execute until after the URB has completed. Otherwise the data you hand
back to the user won't be the data coming from the gadget; it will be
whatever old data was sitting in buf.
> this is the same algorithm for skel_write.so can i make the same algorithm
> with
> skel_read?or not?
Of course not. Reads are different from writes. A write takes data from
the user and then sends it to the device. A read takes data from the
device and then sends it to the user.
Alan Stern
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