Subject: Re: [PATCH v5 3/4] PCI: Add a new option for resource_alignment to reassign alignment

From: Bjorn Helgaas <[email-masked]>
Date: Wed, 28 Sep 2016 17:42:46 -0500

On Tue, Sep 13, 2016 at 05:00:33PM +0800, Yongji Xie wrote:
> When using resource_alignment kernel parameter, the current
> implement reassigns the alignment by changing resources' size
> which can potentially break some drivers. For example, the driver
> uses the size to locate some register whose length is related
> to the size.
> 
> This patch adds a new option "noresize" for the parameter to
> solve this problem.
> 
> Signed-off-by: Yongji Xie <[email-masked]>
> ---
>  Documentation/kernel-parameters.txt |    9 ++++++---
>  drivers/pci/pci.c                   |   37 +++++++++++++++++++++++++----------
>  2 files changed, 33 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index a4f4d69..d6a340d 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -3023,9 +3023,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  				window. The default value is 64 megabytes.
>  		resource_alignment=
>  				Format:
> -				[<order of align>@][<domain>:]<bus>:<slot>.<func>[; ...]
> -				[<order of align>@]pci:<vendor>:<device>\
> -						[:<subvendor>:<subdevice>][; ...]
> +				[<order of align>@][noresize@][<domain>:]
> +				<bus>:<slot>.<func>[; ...]
> +				[<order of align>@][noresize@]pci:<vendor>:<device>
> +				[:<subvendor>:<subdevice>][; ...]
>  				Specifies alignment and device to reassign
>  				aligned memory resources.
>  				If <order of align> is not specified,
> @@ -3036,6 +3037,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  				instances of a device, the PCI vendor,
>  				device, subvendor, and subdevice may be
>  				specified, e.g., 4096@pci:8086:9c22:103c:198f
> +				noresize: Don't change the resources' sizes when
> +				reassigning alignment.
>  		ecrc=		Enable/disable PCIe ECRC (transaction layer
>  				end-to-end CRC checking).
>  				bios: Use BIOS/firmware settings. This is the
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b8357d7..37f8062 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4946,11 +4946,13 @@ static DEFINE_SPINLOCK(resource_alignment_lock);
>  /**
>   * pci_specified_resource_alignment - get resource alignment specified by user.
>   * @dev: the PCI device to get
> + * @resize: whether or not to change resources' size when reassigning alignment
>   *
>   * RETURNS: Resource alignment if it is specified.
>   *          Zero if it is not specified.
>   */
> -static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
> +static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> +		bool *resize)
>  {
>  	int seg, bus, slot, func, align_order, count;
>  	unsigned short vendor, device, subsystem_vendor, subsystem_device;
> @@ -4974,6 +4976,13 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
>  		} else {
>  			align_order = -1;
>  		}
> +
> +		if (!strncmp(p, "noresize@", 9)) {
> +			*resize = false;
> +			p += 9;
> +		} else
> +			*resize = true;
> +
>  		if (strncmp(p, "pci:", 4) == 0) {
>  			/* PCI vendor/device (subvendor/subdevice) ids are specified */
>  			p += 4;
> @@ -5045,6 +5054,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
>  {
>  	int i;
>  	struct resource *r;
> +	bool resize = true;
>  	resource_size_t align, size;
>  	u16 command;
>  
> @@ -5058,7 +5068,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
>  		return;
>  
>  	/* check if specified PCI is target device to reassign */
> -	align = pci_specified_resource_alignment(dev);
> +	align = pci_specified_resource_alignment(dev, &resize);
>  	if (!align)
>  		return;
>  
> @@ -5086,15 +5096,22 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
>  		}
>  
>  		size = resource_size(r);
> -		if (size < align) {
> -			size = align;
> -			dev_info(&dev->dev,
> -				"Rounding up size of resource #%d to %#llx.\n",
> -				i, (unsigned long long)size);
> +		if (resize) {
> +			if (size < align) {
> +				size = align;
> +				dev_info(&dev->dev,
> +					"Rounding up size of resource #%d to %#llx.\n",
> +					i, (unsigned long long)size);
> +			}
> +			r->flags |= IORESOURCE_UNSET;
> +			r->end = size - 1;
> +			r->start = 0;

Why do you want to keep this code that changes the size of the resource?
Can't we just *always* use IORESOURCE_STARTALIGN as below?  It seems like
that would potentially fix bugs, as you mention.

I think it'd be better if we didn't change the size, even if the user
didn't specify "noresize@".  We wouldn't even need the "noresize@" option
then.

Or is there some reason to keep the resize?  If it's just a question of
being afraid to change the existing behavior because of the risk, I'm
willing to take that risk.

> +		} else {
> +			r->flags &= ~IORESOURCE_SIZEALIGN;
> +			r->flags |= IORESOURCE_STARTALIGN | IORESOURCE_UNSET;
> +			r->start = max(align, size);
> +			r->end = r->start + size - 1;
>  		}
> -		r->flags |= IORESOURCE_UNSET;
> -		r->end = size - 1;
> -		r->start = 0;
>  	}
>  	/* Need to disable bridge's resource window,
>  	 * to enable the kernel to reassign new resource
> -- 
> 1.7.9.5
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-pci" in
> the body of a message to [email-masked]
> More majordomo info at  http://vger.kernel.org/majordomo-info.html