linux-kernel - Re: [PATCH 1/2] [PCI] Check that MWI bit really did get set

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Date:	Sun, 15 Oct 2006 15:45:58 -0700
From:	David Brownell <david-b@...bell.net>
To:	Andrew Morton <akpm@...l.org>
Cc:	alan@...rguk.ukuu.org.uk, matthew@....cx,
	val_henson@...ux.intel.com, netdev@...r.kernel.org,
	linux-pci@...ey.karlin.mff.cuni.cz, linux-kernel@...r.kernel.org,
	gregkh@...e.de
Subject: Re: [PATCH 1/2] [PCI] Check that MWI bit really did get set

> > Most drivers should be able to say "enable MWI if possible, but
> > don't worry if it's not possible".  Only a few controllers need
> > additional setup to make MWI actually work ... if they couldn't
> > do that setup, that'd be worth a warning before they backed off
> > to run in a non-MWI mode.
> > 
> 
> So the semantics of pci_set_mwi() are "try to set MWI if this
> platform/device supports it".

Not what I said ... that's what the _driver_ usually wants to do,
which is different from the step implemented by set_mwi(). 


What Alan Cox said is a better paraphrase:

> MWI is an "extra cheese" option not a "no pizza" case

Or "sorry, that car is not available in olive, just burgundy."


Not:

> In that case its interface is misdesigned, because it doesn't discriminate
> between "yes-it-does/no-it-doesn't" (which we don't want to report, because
> either is expected and legitimate) and "something screwed up", which we do
> want to report, because it is always unexpected.

You mis-understand.  It's completely legit for the driver not to care.

I agree that set_mwo() should set MWI if possible, and fail cleanly
if it couldn't (for whatever reason).  Thing is, choosing to treat
that as an error must be the _driver's_ choice ... it'd be wrong to force
that policy into the _interface_ by forcing must_check etc.

- Dave


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