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

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Date:	Fri, 18 Jan 2013 11:16:27 -0500
From:	Don Dutile <ddutile@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
CC:	linux-kernel@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Khalid Aziz <khalid.aziz@...com>
Subject: Re: [PATCH 4/5] PCI: don't touch enable_cnt in pci_device_shutdown()

On 01/18/2013 06:42 AM, Konstantin Khlebnikov 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>

Hmmm.... wondering if this was the problem why kexec folks
said that device_shutdown() didn't work on all systems (when
trying to stop DMA, esp. on IOMMU-enabled systems...).

Bjorn: do you have a list &/or contact in kexec space to try this
patch vs the "reset every PCI bus" strategy that is currently
being pushed for kexec's method to halt DMA from a PCI device ?

> ---
>   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);
>   }
>
>   #ifdef CONFIG_PM
>
> --
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