linux-kernel - Re: [PATCH] usb-musb: keep VBUS on when device is disconnected

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Message-Id: <00882633-3F61-4010-A4D2-D04473282B10@tecnorama.it>
Date:   Mon, 27 Mar 2017 14:53:27 +0200
From:   Moreno Bartalucci <moreno.bartalucci@...norama.it>
To:     Lars Melin <larsm17@...il.com>
Cc:     Bin Liu <b-liu@...com>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alessio Igor Bogani <abogani@...nel.org>
Subject: Re: [PATCH] usb-musb: keep VBUS on when device is disconnected

> Il giorno 25 mar 2017, alle ore 08:21, Lars Melin <larsm17@...il.com> ha scritto:
> 
>> 
>> The usb device has been disconnected, why it still cares about VBUS
>> power?
> 
> Morphing devices (3G dongles, wifi dongles, some printers) boots up in install mode, usually only as a virtual cd-rom containing Windows drivers and software.
> [...]
> why a host only configured port should cut the VBUS supply, it could be always on right?
> 

Yes, that’s exactly the problem I tried to solve with this patch.
I have to add that the problem was not there with kernels up to 4.8.17, I started to see it with 4.9 and up.

By git-bisecting kernel sources, it appears this behaviour has been introduced by this commit:

2f3fd2c5bde1f94513c3dc311ae64494085ec371

I also agree that, in my opinion, a host only port should never remove the VBUS supply, as it happens on all the PCs (linux+windows+mac) that I tested until now.

I saw this problem on a beaglebone black. Of course I’m available to do all the tests that you might suggest me to help you better understand the issue.

Moreno Bartalucci