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

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Date:	Mon, 16 Oct 2006 10:44:30 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Andrew Morton <akpm@...l.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Brownell <david-b@...bell.net>, 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

Andrew Morton writes:

> Let me restore the words from my earlier email which you removed so that
> you could say that:
> 
>   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.  

Maybe aliens are firing a ray-gun at the card.  I think it's
fundamentally wrong for the driver to deny service completely because
of a maybe.

Either there was a transient error that only affected the attempt to
set MWI, in which case a printk (inside pci_set_mwi!) is appropriate,
and we carry on.  Or there is a persistent error condition, in which
case the driver will see something else fail soon enough - something
that the driver actually needs to have working in order to operate -
and fail at that point.

For the driver to stop and refuse to go any further because of an
error in pci_set_mwi has far more disadvantages than advantages.

Paul.
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