Re: [PATCH v6 24/24] PCI/sysfs: Limit BAR resize attribute scope to platforms with PCI mmap

Hello,

> The only platform without these aforementioned defines is Alpha, which is
> conventional PCI only and cannot have ReBAR.  So this guard removes dead
> sysfs code on platforms where it can never be executed.

Having a closer look:

  resource_resize_attr_is_visible()
    pci_rebar_get_current_size()      <- returns -ENOTSUPP, so is_visible callback returns 0
      pci_rebar_find_pos()
        pos = pdev->rebar_cap         <- set to 0 on a conventional PCI
        if (!pos)
          return -ENOTSUPP            <- no ReBAR support

The pdev->rebar_cap is set during PCI enumeration:

  pci_init_capabilities()
    pci_rebar_init()
      pdev->rebar_cap = pci_find_ext_capability()
        pci_find_next_ext_capability()
          if (dev->cfg_size <= PCI_CFG_SPACE_SIZE)
            return 0;                 <- dev->cfg_size set to 256 here for conventional PCI

The PCI_CFG_SPACE_SIZE is 256 here.

When the platform has support for PCI Express, the dev->cfg_size is then
set to 4096.  On an architecture that supports conventional PCI only (such
as Alpha), the pdev->rebar_cap will be set to 0, the is_visible callback
will then return 0, and the resize sysfs attribute is never created, as
such, the __resource_resize_store() callback will never be executed.

To make the connetion here to the #ifdef guards:

For the pci_rebar_get_current_size() to return >= 0, the device needs
PCI Express and extended configuration space support.  As of today,
every architecture with PCI Express support defines HAVE_PCI_MMAP or
ARCH_GENERIC_PCI_MMAP_RESOURCE.

I hope the reasoning here works.

Thank you!

	Krzysztof