Re: [PATCH v9 4/5] xhci: mediatek: support MTK xHCI host controller

[Mathias Nyman]

On 20.10.2015 09:29, chunfeng yun wrote:
> hi,
> On Mon, 2015-10-19 at 14:25 +0300, Mathias Nyman wrote:
> > > > So basically we are trying to use as many microframes as possible with as few packets
> > > > per microframe as possible.
> > > >
> > > > Did I understand this correctly?
> > > Yes, you are right.
> > >
> > > > How will devices react if they expect to get 16 packets every 16th microframe,
> > > > but they get one packet every microframe instead?
> > > I think that the synchronous endpoint must specify its period by
> > > bInterval, but can't specify how data should be transfered during the
> > > period by the host, and it just only receives data passively. So the
> > > device can receive data correctly in the case(bInterval is 5).
> > >
> > > quote from usb3_r1.0 section4.4.8 Isochronous Transfers:
> > > "The host can request data from the device or send data to the device at
> > > any time during the service interval for a particular endpoint on that
> > > device"
> >
> > As I understand the 4.4.8 section it just means the device can't assume a fixed
> > time interval between transfers, meaning that the host can use the last microframe
> > in one esit and the first microframe in the next esit, but still only use 1 microframe
> > per esit.
> >
> > Section 8.12.6.1 describes how a 11 packet isoc transfer is allowed to be split
> > to 1 burst of 11 packets, 2 burst (8 + 3),  3 burst (4+4+3) 6 bursts (2+2+2+2+2+1) or
> > 11 bursts of 1. These are however all within the same microframe. Splitting the
> > transfer into several microframes in a esit kind of makes the whole interval concept pointless.
> It doesn't say that the packets should be transfered within the same
> microframe (bus interval), as I understand it means service interval;
>
> The direct prove resides in figure 8-56/8-57.
>
> Term:
> 1. BI, bus interval, a 125 us period that establishes the internal
> boundary of service interval, aka uframe;
> 2. SSI, Support Smart Isochronous;
> 3. DBI, Data in this Bus Interval is done;
> 4. NBI, Numbers of Bus Interval;
>
> As the figure shows, the service interval = 8 BI, that host distribute 2
> packets @1st uframe, keep U1/U2 state for the next 3uframe, then
> transmit 4 packets @4th uframe, and the remaining 3 packet in the last
> frame.
>
> Please notice that this just is an example illustrated by spec, but we
> can derive the conclusion that the distribution of packet in a service
> interval is completely decided by host, and can split isoc transfers
> across multiple uframes.

So it seems. You're right

> PS: as you can see, MTK implementation of schedule algorithms is an
> implementation of Smart Isochronous of which the smart side resides in
> software.

Thanks for the clarification, I now understand how the implementation works

-Mathias  

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