linux-kernel - Re: [PATCH-RFC 1/2] tile: don't panic on iomap

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Message-ID: <CAErSpo4N8drqe5bHVjzXy_h0HTC8Reye4+8viN4c6TtmwFheXA@mail.gmail.com>
Date:	Tue, 29 Nov 2011 13:04:12 -0800
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Chris Metcalf <cmetcalf@...era.com>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH-RFC 1/2] tile: don't panic on iomap

On Tue, Nov 29, 2011 at 10:54 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> I think panic on iomap is there just for debugging.
> If we return NULL instead, the generic pci_iomap will
> DTRT so we don't need to roll our own.

Just to be explicit about what "doing the right thing" means, here's
what I think is changing (I think the new behavior is OK, but it *is*
different):

Old behavior: Caller calls pci_iomap(), which panics in ioport_map().

New behavior: Caller calls pci_iomap(), ioport_map() returns NULL,
pci_iomap() returns NULL (failure), caller may check for failure.  If
caller does not check for failure and passes the NULL to
ioread()/iowrite(), we WARN in bad_io_access().

>  static inline void __iomem *ioport_map(unsigned long port, unsigned int len)
>  {
> -       return (void __iomem *) ioport_panic();
> +       pr_info("Trying to map an IO resource - it does not exit on tile.\n");
> +       return NULL;

s/exit/exist/

Since we only expect to see this message during debugging, maybe it
could be more informative, e.g., use dump_stack() to identify the
offending driver?  I don't think either the "Trying to map" message or
the "Bad IO access" message is enough to actually make progress in
debugging.

Bjorn
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