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

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, May 06, 2026 at 08:42:00AM +0900, Krzysztof Wilczyński wrote:
> > > > 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.
> > 
> > We're talking about this #ifdef:
> > 
> >   +#if defined(HAVE_PCI_MMAP) || defined(ARCH_GENERIC_PCI_MMAP_RESOURCE)
> >    static ssize_t __resource_resize_show(struct device *dev, int n, char *buf)
> >    ...
> >   +#endif
> > 
> > I follow the reasoning now but by next week I won't, so I think it
> > requires too much background knowledge.  Future changes involving
> > HAVE_PCI_MMAP or ARCH_GENERIC_PCI_MMAP_RESOURCE could easily break
> > this.
> > 
> > IIUC it's basically doing what "#ifdef CONFIG_PCI_REBAR" or even
> > "#ifdef CONFIG_PCI_EXPRESS" would do, if we had such a thing.
> 
> We don't sadly have such guards at the moment, so using the guards like
> HAVE_PCI_MMAP or ARCH_GENERIC_PCI_MMAP_RESOURCE would be the next best
> thing.  We already rely on these throughout the pci-sysfs.c already, and
> will also rely on either for the static sysfs attributes, so if there was
> some issues with either of these, if these "broke" somehow, then we would
> have other more severe problems.  Note, that architectures rely on these
> macros to let us know about what "feature" is expected to be enabled, so to
> speak, so I would imagine nobody would break this accidentally (famous last
> words) and perhaps with extreme caution would do it on purpose.
>
> > How terrible would it be if we just accepted this dead code on Alpha?
> 
> I would prefer not to build any dead code anywhere if it can be helped,
> especially with as little as two lines of code added.  Plus, most of the
> code setting up resources that are not specific to Alpha, would have been
> disabled using exactly the same guard as the one proposed to be added
> here, not like this sets any precedent or adds something new.

OK, let's leave it as-is.  It's confusing to read, but no more so than
other existing uses.