Re: [Qemu-devel] [PATCH 1/2] hw/usb-ohci: Honour endpoint maximum packet size

[Gerd Hoffmann]

  Hi,

> > No.  What I think is that USBPacket shouldn't be required to be an actual
> > USB packet, but a transfer, i.e. do the splitting of larger transfers into
> > smaller packets in the usb driver emulation (if needed), not the host
> > adapter emulation.
>
> The OHCI spec still requires us to only process one max-packet-size
> worth of data from this TransferDescriptor before we move on to
> the next one, though, doesn't it? (cf the flowchart in fig 6-7).

[ didn't look at specs ]

Yes, EHCI has a simliar rule, on real hardware it is needed to ensure 
the available USB bandwidth is fairly splitted across all devices by 
serving them in a robin-round fashion.

I don't think this makes much sense for emulation as it behaves very 
different compared to real hardware.  There simply is no bus with 
limited bandwidth we have to care about.

> It seems to me that at least some of that is likely to be
> guest-visible, especially in the case where an endpoint returns an
> error partway through. So it's not clear to me that you could
> validly batch up everything in the TD and do it all at once.

I think the only guest-visible effect is that the guest will never ever 
see an partially filled transfer block, it is either empty or completely 
filled (except on short transfers).

cheers,
  Gerd