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Subject: Re: [PATCH] usb/host/pci-quirks: Only reset USB bus on NVIDIA devices
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
References: <Pine.LNX.4.44L0.1807021135320.1574-100000@iolanthe.rowland.org>
From:   Paul Menzel <pmenzel@molgen.mpg.de>
Message-ID: <3301ef51-e35f-1273-6efc-8c11f31a945c@molgen.mpg.de>
Date:   Mon, 2 Jul 2018 17:52:43 +0200
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[Taking stable@ out of CC. I just started to use `git send-email`.]


Dear Alan,


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.

> 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.


Kind regards,

Paul