Re: [PATCH] usb/host/pci-quirks: Only reset USB bus on NVIDIA devices

[Paul Menzel]

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 itâs 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, itâs 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?


Kind regards,

Paul