Date:	Thu, 30 Apr 2015 16:49:06 +0800
From:	Zhuang Jin Can <jin.can.zhuang@...el.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	rafael.j.wysocki@...el.com, stern@...land.harvard.edu,
	dan.j.williams@...el.com, pmladek@...e.cz,
	peter.chen@...escale.com, jwerner@...omium.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH] usb: core: add usb3 lpm sysfs

On Wed, Apr 29, 2015 at 09:57:33PM +0200, Greg KH wrote:
> On Thu, Apr 30, 2015 at 12:21:32AM +0800, Zhuang Jin Can wrote:
> > On Wed, Apr 29, 2015 at 01:06:22PM +0200, Greg KH wrote:
> > > On Wed, Apr 29, 2015 at 06:57:30PM +0800, Zhuang Jin Can wrote:
> > > > On Wed, Apr 29, 2015 at 11:01:48AM +0200, Greg KH wrote:
> > > > > On Wed, Apr 29, 2015 at 03:20:04PM +0800, Zhuang Jin Can wrote:
> > > > > > On Tue, Apr 28, 2015 at 11:11:10PM +0200, Greg KH wrote:
> > > > > > > On Wed, Apr 29, 2015 at 12:51:27AM +0800, Zhuang Jin Can wrote:
> > > > > > > > Hi Greg KH,
> > > > > > > > 
> > > > > > > > On Tue, Apr 28, 2015 at 12:42:24PM +0200, Greg KH wrote:
> > > > > > > > > On Sun, Apr 19, 2015 at 11:46:12AM +0800, Zhuang Jin Can wrote:
> > > > > > > > > > Some usb3 devices may not support usb3 lpm well.
> > > > > > > > > > The patch adds a sysfs to enable/disable u1 or u2 of the port.The
> > > > > > > > > > settings apply to both before and after device enumeration.
> > > > > > > > > > Supported values are "0" - u1 and u2 are disabled, "u1" - only u1 is
> > > > > > > > > > enabled, "u2" - only u2 is enabled, "u1_u2" - u1 and u2 are enabled.
> > > > > > > > > > 
> > > > > > > > > > The interface is useful for testing some USB3 devices during
> > > > > > > > > > development, and provides a way to disable usb3 lpm if the issues can
> > > > > > > > > > not be fixed in final products.
> > > > > > > > > 
> > > > > > > > > How is a user supposed to "know" to make this setting for a device?  Why
> > > > > > > > > can't the kernel automatically set this value properly?  Why does it
> > > > > > > > > need to be a kernel issue at all?
> > > > > > > > > 
> > > > > > > > By default kernel enables u1 u2 of all USB3 devices. This interface
> > > > > > > > provides the user to change this policy. User may set the policy
> > > > > > > > according to PID/VID of uevent or according to the platform information
> > > > > > > > known by userspace.
> > > > > > > 
> > > > > > > And why would they ever want to do that?
> > > > > > > 
> > > > > > > > It's not a kernel issue, as u1 u2 is mandatory by USB3 compliance. But
> > > > > > > > for some internal hardwired USB3 connection, e.g. SSIC, passing USB3
> > > > > > > > compliance is not mandatory. So the interface provides a way for vendor
> > > > > > > > to ship with u1 or u2 broken products. Of course, this is not encouraged :).
> > > > > > > 
> > > > > > > If the state is broken for those devices, we can't require the user to
> > > > > > > fix it for us, the kernel should do it automatically.
> > > > > > > 
> > > > > > > > > And when you are doing development of broken devices, the kernel doesn't
> > > > > > > > > have to support you, you can run with debugging patches of your own
> > > > > > > > > until you fix your firmware :)
> > > > > > > > > 
> > > > > > > > Understood. But I think other vendor or developer may face the same
> > > > > > > > issue in final product shipment or during development. Moreover, the
> > > > > > > > interface provide the flexibility for developer to separately
> > > > > > > > disable/enable u1 or u2, e.g. If they're debugging an u2 issue, they
> > > > > > > > can disable u1 to simplify the situtation.
> > > > > > > 
> > > > > > > For debugging only, perhaps, but for a "normal" user, please let's
> > > > > > > handle this automatically and don't create a switch that never gets used
> > > > > > > by anyone or anything.
> > > > > > > 
> > > > > > Thanks Greg. Since so far the patch has no interesting value to the
> > > > > > community, I'll drop the patch.
> > > > > 
> > > > > I didn't say that, I said it needed some more work to be accepted.
> > > > Sorry for misunderstanding. Let me explain more why we need this interface.
> > > > 
> > > > We have a modem USB3 device (in stepping C) hardwired to one specific port of xHCI.
> > > > The device was expected to work with u1 u2, however, due to a HW issue, it doesn't
> > > > work stably. To workaround the issue, we let the init.rc script disable u1 u2 for
> > > > this specific port.
> > > 
> > > Modern Linux systems don't have init.rc scripts anymore :)
> > > 
> > In Android, the init process still reads an init.rc where vendor can
> > define their own policies. Vendors normally provides a whole reference
> > design (including HW, FW, Kernel, BSP, AOSP) to OEMs. BSP contains
> > vendor specific configurations including its own init.rc.
> 
> And that's generally not a good idea for companies to do, as they
> shouldn't need special hardware workarounds in an init script, but I
> understand :(
> 
> You are also going to be giving them a kernel patch that is not accepted
> upstream which is really NOT the way to do things, and something that
> many of us are working quite hard to keep from happening.
> 
> > > > Then maybe we want to start debug u1 issue first, to avoid hitting u2 issue,
> > > > we can disable u2. After u1 issue is resolved, we can enable back u2 to continue to
> > > > debug u2 issue. This provides the flexibility to isolate u1 u2 debugging.
> > > > This is valuable I think :)
> > > 
> > > I agree.
> > > 
> > > > The HW issue will be fixed in stepping D, however C and D will have the same PID/VID.
> > > > There's no way for kernel to know the difference between C and D.
> > > > Even after fixing in D, C will still be used for development (to save money..)
> > > 
> > > That sounds like a big design flaw, what about looking at the version of
> > > the device?  That's what that field in the USB descriptors is for.
> > > 
> > You're right. bcdDevice should be used for this purpose.
> 
> Why can't it?
> 
> > However, since we need to live with what we have, we just used the init.rc to disable
> > u1/u2. Definitely, it's a ugly hack in userspace to make it "automatically" work.
> 
> Why can't you put a quirk in the kernel for that bcdDevice value and
> then not need any userspace hacks?
> 
Let me double confirm with our modem HW engineers to see if anything we
can differentiate the modem variants, and add the quirks in kernel
accordingly.

> > A better solution is to use monitor uevent, reading bcdDevice + PID/VID, and define
> > a rule in to disable u1/u2 of this device. Android provides an uevent machanism to do this.
> 
> It's a horrid uevent mechanism in that it duplicates what udev did years
> ago :(
> 
OK. I think I got your point now: kernel should handle broken devices automatically without
userspace's attention.


> > However, how to do it automatically, it's out of the scope of the patch.
> 
> Not at all, what if you don't want to run Android on your hardware?  You
> still want it to work, so get the kernel fix upstream properly.
> 
Got it.

> > Without the patch, the only choice is to add a quirk in usb core to do it automatically. And
> > this should be in another separate patch.
> 
> Please send that patch.
> 
Sure. Will need some time before sending the patch.

> > With the patch:
> > 	1. Userspace can also do the quirk with the help of uevent and rules
> 
> But it has no idea how or when to do that.  Please don't provide hooks
> that no one knows how to use.
> 
OK.

> > 	2. Developer can isolate u1 u2 debugging.
> 
> That only developer seems to be you, and you've already debugged this :)
> 
Do you think the interface has no much value to other developers, and I should
remove it?

> > And I don't think it's necessary for kernel to support this broken modem. Because, the modem
> > is integrated with the SoC, and SoC goes with init.rc to OEMs. Thus, it doesn't make sense we
> > add a quirk in kernel to long term support it. The SoC/Modem is going to be replaced by its next
> > generation, especially in mobile area.
> 
> Again, someone wants to run a mainline kernel.org release on that
> hardware, like they should.  Some companies even are pushing to require
> OEMs to have all of their changes upstream before they will buy their
> chip, so please, make this a quirk and have it "just work" properly.  No
> need to rely on a magic init.rc value that no one notices or
> understands.
> 
Got it. Thanks your explainations Greg.

Regards
Jincan
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