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

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Date:	Sun, 15 Oct 2006 16:18:34 -0700
From:	Andrew Morton <akpm@...l.org>
To:	David Brownell <david-b@...bell.net>
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

On Sun, 15 Oct 2006 15:45:58 -0700
David Brownell <david-b@...bell.net> wrote:

> > 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.

No.  If pci_set_mwi() detects an unexpected error then the driver should
take some action: report it, recover from it, fail to load, etc.  If the
driver fails to do any of this then it's a buggy driver.

You, the driver author _do not know_ what pci_set_mwi() does at present, on
all platforms, nor do you know what it does in the future.  For you the
driver author to make assumptions about what's happening inside
pci_set_mwi() is a layering violation.  Maybe the bridge got hot-unplugged.
 Maybe the attempt to set MWI caused some synchronous PCI error.  For
example, take a look at the various implementations of pci_ops.read()
around the place - various of them can fail for various reasons.  

Now it could be that an appropriate solution is to make pci_set_mwi()
return only 0 or 1, and to generate a warning from within pci_set_mwi()
if some unexpected error happens.  In which case it is legitimate for
callers to not check for errors.

This is not a terribly important issue, and it is far from the worst case
of missed error-checking which we have in there. 
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