linux-kernel - Re: [RFC] usb: dwc2: hcd: fix split schedule issue

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Message-ID: <CAODwPW8b3amZT1S+x6McngXPd5kZVU7_Gz=U3-Wftov-ZUfdSg@mail.gmail.com>
Date:	Mon, 16 Nov 2015 12:46:32 -0800
From:	Julius Werner <jwerner@...gle.com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	John Youn <John.Youn@...opsys.com>, Yunzhi Li <lyz@...k-chips.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	"Kaukab, Yousaf" <yousaf.kaukab@...el.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	wulf <wulf@...k-chips.com>,
	Kever Yang <kever.yang@...k-chips.com>,
	caesar <caesar.wang@...k-chips.com>,
	Tao Huang <huangtao@...k-chips.com>,
	rockchip-discuss <rockchip-discuss@...omium.org>
Subject: Re: [RFC] usb: dwc2: hcd: fix split schedule issue

> To handle things smarter, I think I need to research how to deal with
> hubs attached to hubs attached to hubs.  For instance:
>
> dwc2
> -> multi_tt hub
>     -> single_tt hub
>         -> device 1
>         -> device 2

Keep in mind that there's always at most one (active) TT between host
and device. The TT is the point where high-speed traffic is translated
to low-/full-speed traffic, so after that you cannot translate again.
With multiple hubs you either have

-> high-speed 2.0 hub (TT inactive / irrelevant for this path)
  -> multi or single TT 2.0 hub
    -> device

or

-> multi or single TT 2.0 hub
  -> full-speed 1.1 hub
     -> device

All the information you need should already be in struct usb_device.
If udev->tt->multi == 0, then it must be scheduled in the same group
as all other devices it shares udev->tt (the same pointer address)
with. If udev->tt->multi == 1, then it belongs in the same group as
all that have the same udev->tt and the same udev->ttport. There's
even a udev->tt->hcpriv where you could link a data structure (array)
in to keep track of these matching devices.

I agree that this is a nice-to-have optimization, though... it's more
important to get the thing stable, and I think it's fine to assume
that all low-/full-speed transfers go on the same bus for the first
iteration.
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