Re: [PATCH] usb: core: add usb3 lpm sysfs

From: Greg KH
Date: Tue Apr 28 2015 - 17:11:20 EST

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.


greg k-h
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