linux-kernel - Re: [PATCH 4/5] PCI: don't touch enable_cnt in pci_device

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Date:	Mon, 28 Jan 2013 16:40:13 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	linux-pci@...r.kernel.org, Khalid Aziz <khalid.aziz@...com>
Subject: Re: [PATCH 4/5] PCI: don't touch enable_cnt in pci_device_shutdown()

On Fri, Jan 18, 2013 at 4:42 AM, Konstantin Khlebnikov
<khlebnikov@...nvz.org> wrote:
> comment in commit b566a22c23327f18ce941ffad0ca907e50a53d41
> ("PCI: disable Bus Master on PCI device shutdown") says:
>
> | Disable Bus Master bit on the device in pci_device_shutdown() to ensure PCI
> | devices do not continue to DMA data after shutdown.  This can cause memory
> | corruption in case of a kexec where the current kernel shuts down and
> | transfers control to a new kernel while a PCI device continues to DMA to
> | memory that does not belong to it any more in the new kernel.
>
> Seems like pci_clear_master() must be used here instead of pci_disable_device(),
> because it disables Bus Muster unconditionally and doesn't changes enable_cnt.
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> Cc: linux-pci@...r.kernel.org
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: Khalid Aziz <khalid.aziz@...com>
> ---
>  drivers/pci/pci-driver.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index 030dbf0..853d605 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -392,7 +392,7 @@ static void pci_device_shutdown(struct device *dev)
>          * Turn off Bus Master bit on the device to tell it to not
>          * continue to do DMA
>          */
> -       pci_disable_device(pci_dev);
> +       pci_clear_master(pci_dev);

We currently only call pci_enable_device() and pci_disable_device()
from drivers, and I think that's a nice division that's worth keeping.
 It keeps the core's mitts off device operation and helps preserve the
enable_cnt integrity.  So I like this change from that perspective.

Any objections to this, Khalid?

>  }
>
>  #ifdef CONFIG_PM
>
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