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

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Date:	Sun, 15 Oct 2006 16:44:02 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	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

On Mon, 16 Oct 2006 01:02:40 +0100
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> Ar Sul, 2006-10-15 am 16:18 -0700, ysgrifennodd Andrew Morton:
> > 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.
> 
> Wrong and there are several drivers in the kernel that are proof of
> this.

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.  

> > 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
> 
> You don't care. It isn't an error for set_mwi to fail. In fact the only
> reason set_mwi even needs to bother with a return code is that some
> chips want you to set other config private to the device if it is
> available and active.
> 

Let me restore the words from my earlier email which you removed which
address that:

  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.

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