linux-kernel - Re: [PATCH] usb: core: add usb3 lpm sysfs

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Message-ID: <20150428165127.GA26638@intel.com>
Date:	Wed, 29 Apr 2015 00:51:27 +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

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.

Thanks
Jincan
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