Re: [PATCH 02/10] xhci: 'noxhci_port_switch' kernel parameter

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, May 20, 2014 at 03:40:16PM -0700, Dan Williams wrote:
> On Tue, May 20, 2014 at 1:34 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Tue, May 20, 2014 at 11:25:37AM -0700, Dan Williams wrote:
> >> On Tue, May 20, 2014 at 2:51 AM, Takashi Iwai <tiwai@suse.de> wrote:
> >> > At Tue, 20 May 2014 12:47:36 +0300,
> >> > Mathias Nyman wrote:
> >> >>
> >> >> On 05/20/2014 04:01 AM, Greg KH wrote:
> >> >> > On Thu, May 08, 2014 at 07:25:55PM +0300, Mathias Nyman wrote:
> >> >> >> From: Dan Williams <dan.j.williams@intel.com>
> >> >> >>
> >> >> >> Add a command line switch for disabling ehci port switchover.  Useful
> >> >> >> for working around / debugging xhci incompatibilities where ehci
> >> >> >> operation is available.
> >> >> >>
> >> >> >> Reference: http://marc.info/?l=linux-usb&m=138920063001509&w=2
> >> >> >>
> >> >> >> Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
> >> >> >> Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
> >> >> >> Cc: Holger Hans Peter Freyther <holger@moiji-mobile.com>
> >> >> >> Suggested-by: Alan Stern <stern@rowland.harvard.edu>
> >> >> >> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> >> >> >> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
> >> >> >> ---
> >> >> >>  Documentation/kernel-parameters.txt |  3 +++
> >> >> >>  drivers/usb/host/pci-quirks.c       | 15 +++++++++++++--
> >> >> >>  2 files changed, 16 insertions(+), 2 deletions(-)
> >> >> >>
> >> >> >> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> >> >> >> index 4384217..fc3403114 100644
> >> >> >> --- a/Documentation/kernel-parameters.txt
> >> >> >> +++ b/Documentation/kernel-parameters.txt
> >> >> >> @@ -2251,6 +2251,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >> >> >>
> >> >> >>    nox2apic        [X86-64,APIC] Do not enable x2APIC mode.
> >> >> >>
> >> >> >> +  noxhci_port_switch
> >> >> >> +                  [USB] Use EHCI instead of XHCI where available
> >> >> >> +
> >> >> >
> >> >> > Ugh, I really don't like new command line options.
> >> >> >
> >> >> > Especially one that isn't very well documented.  Why would someone want
> >> >> > to enable this?  What problem is it solving?  Can we detect this
> >> >> > automatically and do it for the user?  Is this just for prototype
> >> >> > hardware that has not shipped?  What hardware needs this?
> >> >> >
> >> >> > I need a whole lot more documentation at the very least before I will
> >> >> > apply this.
> >> >> >
> >> >>
> >> >> On Intel hardware with both ehci and xhci controllers we can select if a usb2 port
> >> >> is controlled by ehci or xhci. This capability can be checked from Intel xhci pci
> >> >> config space. Xhci driver checks this on boot and switches over the supported ports.
> >> >>
> >> >> This is a feature in Intel Panther point and later chipsets, in shipped hardware.
> >> >> Its working quite well in most cases, but sometimes vendors claim they support
> >> >> switchover, but then forget to connect some wires, and the usb2 port ends up dead
> >> >> after switching.
> >> >>
> >> >> A recently found case is the Sony VAIO T-series. (I'll send you a different patch
> >> >> for that shortly)
> >> >> http://marc.info/?l=linux-usb&m=139993106029340&w=2
> >> >>
> >> >> This is the extreme case that the usb2 ports appears completely dead.
> >> >> Other reasons are that some devices might work better under ehci than xhci,
> >> >> and users want to enforce the ehci opton. For powermanagement developers it's nice
> >> >> to disable switchover as it turns out some hardware are quirky with port
> >> >> switchover and suspend/resume. (might need to turn port back to ehci before
> >> >> suspending).
> >> >>
> >> >> I don't think we can detect this automatically.
> >> >>
> >> >> Dan, can you add more documentation to this patch?
> >> >
> >> > While we're at it: can we implement a bitmask instead?
> >> >
> >> > We've seen lots of HP laptops having Webcams or BT devices that don't
> >> > work XHCI but only with EHCI.  For making them working properly, the
> >> > specific xhci ports have to be disabled.  But, we don't want to kill
> >> > the whole XHCI at all.  The single boolean option doesn't work for
> >> > such a case.
> >> >
> >>
> >> I'm not sure we want to make this more complex.  Ideally this is just
> >> a stop-gap measure for users to workaround incompatibilities in xhci
> >> while the xhci fix is identified.
> >
> > We can't use a kernel command line option as a "stop-gap", sorry.  Let's
> > just fix the real problem here.
> >
> 
> Greg,
> 
> Sorry, I don't think it is fair to users to force them to re-compile
> their kernel to get their device to work.

I totally agree.

> Granted, I'm new to USB
> development, but the rate of reports of endpoint devices that mess up
> and require quirks in the hcd-driver or usb-core seems un-ending to
> me.  So, I don't think it is fair to expect that the tide of quirky
> devices will be stemmed in any reasonable amount of time.  Having a
> "works with noxhci_port_switch" report from users is good data (hmm, I
> think a printk to tell users to file a report upstream if the option
> resolves their issue is needed).

How about just adding a debugfs file instead?  That way, once you fix
this, we can then remove it and no one will care.

thanks,

greg k-h