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

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Date:	Tue, 20 Oct 2015 11:25:23 +0300
From:	Mathias Nyman <mathias.nyman@...el.com>
To:	chunfeng yun <chunfeng.yun@...iatek.com>
CC:	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Felipe Balbi <balbi@...com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Roger Quadros <rogerq@...com>, linux-usb@...r.kernel.org,
	linux-mediatek@...ts.infradead.org,
	John Crispin <blogic@...nwrt.org>,
	Daniel Kurtz <djkurtz@...omium.org>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Kishon Vijay Abraham I <kishon@...com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v9 4/5] xhci: mediatek: support MTK xHCI host controller

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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