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Date:   Mon, 2 Jul 2018 12:34:49 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Paul Menzel <pmenzel@molgen.mpg.de>
cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        <linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] usb/host/pci-quirks: Only reset USB bus on NVIDIA devices
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On Mon, 2 Jul 2018, Paul Menzel wrote:

> Dear Alan,
> 
> 
> Am 02.07.2018 um 18:03 schrieb Alan Stern:
> > On Mon, 2 Jul 2018, Paul Menzel wrote:
> 
> >> Am 02.07.2018 um 17:45 schrieb Alan Stern:
> >>> On Sun, 1 Jul 2018, Paul Menzel wrote:
> >>>
> >>>> Currently, on the AMD board Asus F2A85-M Pro there is a 100 ms delay as
> >>>> the USB bus of each of the two OHCI PCI devices is reset. As a 50 ms
> >>>> delay is done per the USB specification.
> >>>>
> >>>> Commit c6187597 (OHCI: final fix for NVIDIA problems (I hope))
> >>>> unconditionally does the bus reset for
> >>>>    all chipsets, while it was only doen for NVIDIA chipsets before.
> >>>
> >>> I don't follow this at all.  Prior to that commit, the bus reset (i.e.,
> >>>
> >>> 	writel(control & OHCI_CTRL_MASK, base + OHCI_CONTROL);
> >>>
> >>> ) was performed unconditionally for _all_ controllers.  (However, the
> >>> 50-ms delay was used only for NVIDIA hardware.)  Following that commit,
> >>> the reset is performed for all controllers, but only if the HCFS
> >>> bitfield is nonzero.
> >>>
> >>>> As it should not be needed for non-NVIDIA chipsets, only do the reset
> >>>> for Nvidia devices.
> >>>
> >>> Therefore this reasoning is wrong.
> >>
> >> True. Thank you for checking that.
> >>
> >>>> Tested on Asus F2A85-M PRO and ASRock E350M1. The USB keyboard works and
> >>>> the LUKS passphrase can be entered.
> >>>
> >>> Unfortunately, there is a wide variety of OHCI controller hardware
> >>> available.  Something that works on one or two controllers might not
> >>> work on another.
> >>
> >> The problem is, that currently 100 ms sleep is over 10 % of the overall
> >> execution time of the Linux kernel here. So I really like to not sleep
> >> if it’s not needed.
> > 
> > It would be nice to execute the probes in parallel; that would reduce
> > the total delay to 50 ms.  However, that is the subject of a separate
> > discussion.
> > 
> >>> Besides, doesn't it seem like a bad idea to reset the controller while
> >>> leaving devices on the USB bus in whatever state they happened to be?
> >>
> >> Yes, it’s probably not optimal.
> >>
> >> I wonder if the reset is needed, if the firmware has already initialized
> >> the device.
> > 
> > The devices on the bus need to be reset, because the OS has no idea of
> > what they are and what the firmware has them doing.
> > 
> > For example, the firmware may have assigned bus address 2 to a
> > keyboard.  But the OS can initialize the devices in a different order,
> > and it might want to assign bus address 2 to a USB drive.  Then you'd
> > have two devices trying to use the same address at the same time, which
> > would not be a good thing.
> 
> Understood.
> 
> So, what would be a way forward? Add a whitelist for boards or chipsets
> not needing the 50 ms delay?

The 50-ms delay isn't for the board or the chipset; it is for the USB 
devices that are plugged into the controller.  It is required by the 
USB spec, so we can't get rid of it.

Alan Stern