linux-kernel - Re: [PATCH 2/4] [USB] UASP: Add MaxNumStreams module parameter

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Date:	Thu, 9 Dec 2010 16:41:46 -0800 (PST)
From:	Luben Tuikov <ltuikov@...oo.com>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Greg KH <greg@...ah.com>
Cc:	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 2/4] [USB] UASP: Add MaxNumStreams module parameter

--- On Thu, 12/9/10, Greg KH <greg@...ah.com> wrote:
> -0800, Sarah Sharp wrote:
> > On Thu, Dec 09, 2010 at 02:54:19PM -0800, Luben Tuikov
> wrote:
> > > --- On Thu, 12/9/10, Sarah Sharp <sarah.a.sharp@...ux.intel.com>
> wrote:
> > > > -0800, Luben Tuikov wrote:
> > > > > Signed-off-by: Luben Tuikov <ltuikov@...oo.com>
> > > > > ---
> > > > >  The long story is that we see some
> host
> > > > controllers misreport their
> > > > >  PSA as they solved 2^v = streams,
> instead of
> > > > 2^(v+1) = streams. Thus
> > > > >  They report that they support 32
> streams when in
> > > > fact they support 16.
> > > > >  When the device attempts to return
> status for
> > > > stream > 15, the host
> > > > >  says ACK(NumP=0), the device goes in
> flow
> > > > control, blah, blah, this
> > > > >  module parameter allows you to set a
> max cap on
> > > > the number of streams
> > > > >  the driver will ask XHCI HCD to
> allocate.
> > > > 
> > > > If this is an issue with a host then the
> work-around should
> > > > be in the
> > > > xHCI driver instead.  It should be based on
> the
> > > > vendor/device ID of the
> > > > offending host instead of being a module
> parameter in the
> > > > UAS driver.
> > > > The only people who benefit from this patch
> are the people
> > > > "in the know"
> > > > about which hosts are buggy, not normal
> Linux users.
> > > 
> > > The OS might want to limit the number of streams
> for reasons other
> > > than a buggy HC. So this module parameter add
> flexibility other than
> > > making a buggy HC work.
> > 
> > Ok, that's fine, but my comments still stand about
> fixing this
> > particular bug in the xHCI driver, rather than in the
> UAS driver.
> 
> I agree.
> 
> Also, a module parameter will not work when your driver is
> connecting to
> different xHCI devices in the same system.  Please
> never add module
> parameters anymore, there is usually never a need for them
> at all.

I hesitated a lot before adding this module parameter. I did not want to add it since of course the obvious solution is to have a "black list" in the XHCI HCD. I think everyone knows this.

But at the time and still in some ways, it seemed like a valuable addition.  Of course, when not set (default behavior) it doesn't impact the operation of the driver.

   Luben

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