Re: [RESEND PATCH v2 3/4] PCI: Add a new option for resource_alignment to reassign alignment

[Yongji Xie]

On 2016/6/21 9:57, Bjorn Helgaas wrote:

> On Thu, Jun 02, 2016 at 01:46:50PM +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.
> Why do we ever want to change the resource's size?  I understand that
> you want to change the resource's *alignment*, and that part makes
> sense.  But why change the *size*?  Changing the resource size doesn't
> change the hardware BAR size; it just means the struct resource will
> describe a region larger than what the BAR actually claims.  That
> unnecessarily wastes space after the BAR.
>
> This was a problem with the code even before your patch; I'm
> suggesting that if you have a way to change the alignment without
> changing the resource size, maybe we should do that all the time.
> Then you wouldn't need to add the "noresize" option.

Yes, changing resource's size seems not a good idea. But
would it break some existing systems which are using this
kernel parameter if we remove the "noresize" option and change
the alignment all the time?

Thanks,
Yongji

> > Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
> > ---
> >   Documentation/kernel-parameters.txt |    5 ++++-
> >   drivers/pci/pci.c                   |   35 +++++++++++++++++++++++++----------
> >   2 files changed, 29 insertions(+), 11 deletions(-)
> >
> > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > index 82b42c9..c4802f5 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -2997,13 +2997,16 @@ 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>@][<domain>:]<bus>:<slot>.<func>
> > +				[:noresize][; ...]
> >   				Specifies alignment and device to reassign
> >   				aligned memory resources.
> >   				If <order of align> is not specified,
> >   				PAGE_SIZE is used as alignment.
> >   				PCI-PCI bridge can be specified, if resource
> >   				windows need to be expanded.
> > +				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 a259394..3ee13e5 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -4748,11 +4748,13 @@ 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;
> >   	resource_size_t align = 0;
> > @@ -4786,6 +4788,11 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
> >   			}
> >   		}
> >   		p += count;
> > +		if (!strncmp(p, ":noresize", 9)) {
> > +			*resize = false;
> > +			p += 9;
> > +		} else
> > +			*resize = true;
> >   		if (seg == pci_domain_nr(dev->bus) &&
> >   			bus == dev->bus->number &&
> >   			slot == PCI_SLOT(dev->devfn) &&
> > @@ -4818,11 +4825,12 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> >   {
> >   	int i;
> >   	struct resource *r;
> > +	bool resize = true;
> >   	resource_size_t align, size;
> >   	u16 command;
> >   
> >   	/* 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;
> >   
> > @@ -4844,15 +4852,22 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> >   		if (!(r->flags & IORESOURCE_MEM))
> >   			continue;
> >   		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;
> > +		} 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
> >
> > --
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