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

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Date:   Tue, 3 Jul 2018 08:49:40 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb/host/pci-quirks: Only reset USB bus on NVIDIA devices

Dear Alan,


Thank you very much for looking into this issue.

Am 02.07.2018 um 19:19 schrieb Alan Stern:
> On Mon, 2 Jul 2018, Alan Stern wrote:
> 
>>>>> 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.
> 
> I may have been too hasty.  The drivers for other types of USB host
> controllers do not perform a USB bus reset during startup, so maybe
> OHCI doesn't really need it either.
> 
> We still need to put the controller into the correct state, but what
> happens to the bus shouldn't matter.  This is because reinitializing
> the controller disables all its ports, and the only way to enable a
> port is by sending a reset signal through it.  So the attached devices
> will end up getting reset one way or another, even if we don't do it
> explicitly during the handoff.
> 
> Therefore in theory we should be okay without the 50-ms delay at all.
> And we might as well tell the controller to go into the USB_RESET even
> if it already is in that state; testing the current state is more
> effort than just doing setting it.  Something like the patch below
> ought to work, although I wouldn't submit it for the -stable kernels
> since it doesn't fix a real bug.
> 
> It's not clear why OHCI has both a USB_RESET state and a
> HostControllerReset command, especially since the command changes the
> state to USB_SUSPEND instead of USB_RESET.  This is just one of several
> odd things in the OHCI specification.

> Index: usb-4.x/drivers/usb/host/pci-quirks.c
> ===================================================================
> --- usb-4.x.orig/drivers/usb/host/pci-quirks.c
> +++ usb-4.x/drivers/usb/host/pci-quirks.c
> @@ -783,15 +783,9 @@ static void quirk_usb_handoff_ohci(struc
>   	/* disable interrupts */
>   	writel((u32) ~0, base + OHCI_INTRDISABLE);
>   
> -	/* Reset the USB bus, if the controller isn't already in RESET */
> -	if (control & OHCI_HCFS) {
> -		/* Go into RESET, preserving RWC (and possibly IR) */
> -		writel(control & OHCI_CTRL_MASK, base + OHCI_CONTROL);
> -		readl(base + OHCI_CONTROL);
> -
> -		/* drive bus reset for at least 50 ms (7.1.7.5) */
> -		msleep(50);
> -	}
> +	/* Go into the USB_RESET state, preserving RWC (and possibly IR) */
> +	writel(control & OHCI_CTRL_MASK, base + OHCI_CONTROL);
> +	readl(base + OHCI_CONTROL);
>   
>   	/* software reset of the controller, preserving HcFmInterval */
>   	if (!no_fminterval)

Tested on ASRock E350M1 with GRUB as bootloader, and a mouse attached to 
the USB port. The mouse works under X.


Kind regards,

Paul