Re: New USB stack on FreeBSD w/Linux emulation layer coming

On Wed, 16 May 2007, Hans Petter Selasky wrote:

> On Wednesday 16 May 2007 18:25, Alan Stern wrote:
> > On Wed, 16 May 2007, Hans Petter Selasky wrote:
> > > It is very clear to me that non-blocking memory allocation at the point
> > > of starting an USB transfer will require extra error handling in the USB
> > > device driver code!
> >
> > It's not so clear to me.
> 
> In my new USB stack, I pass all endpoint and buffer size information 
> to "usbd_transfer_setup()" at attach time:
> 
> For example:

Irrelevant.  This doesn't affect the need for error handling later on.

> > > My "usbd_transfer_start()" returns "void". Your "usb_submit_urb()"
> > > returns "int".
> >
> > URB submission has other failure possibilities than lack of memory.
> > Those other things have to be checked for regardless.
> 
> Yes, but that is because you allow too many parameters in the URB to be 
> changed between USB transfers.

No; it's because unforeseen events can occur.  For example, the device
may have been unplugged or suspended.

> > > Pre-allocating everything you need simply saves code, hence you only
> > > check once if you got the memory or not.
> >
> > It doesn't save code.  You need to check for the memory when you
> > allocate it, no matter when that is done.
> 
> Yes, but it is a difference doing it once at attach or doing it every time you 
> start a transfer.

Above you said it "saves code".  That is wrong.

> > One allocation = one check. 
> > The total code size is the same if you do the allocation early and only
> > once or if you do it late and many times.
> >
> > It does save execution time.  But that's a different matter; it also
> > wastes data space.
> 
> Yes, it wastes some memory, but that is just how it is. Get used to it :-)
> 
> I have some times been thinking that USB endpoint descriptors should have 
> supplied some information on the maximum bandwidth supported, so that the USB 
> devices can allocate appropriate buffers.

The manufacturers would probably get it wrong anyway.

Alan Stern


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