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

Bjorn Helgaas
2016-06-21T01:58:04Z

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.

> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
> ---
> 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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