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

From: Zhuang Jin Can
Date: Tue Apr 28 2015 - 12:51:39 EST

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.

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 :).

> 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.

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