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

[Greg KH <gregkh@linuxfoundation.org>]

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.

thanks,

greg k-h