Re: [PATCH v4 2/7] PCI: endpoint: Add pci_epc_bar_size_to_rebar_cap()

[Niklas Cassel <cassel@kernel.org>]

On Tue, Feb 18, 2025 at 10:48:04AM -0600, Bjorn Helgaas wrote:
> On Fri, Jan 31, 2025 at 07:29:51PM +0100, Niklas Cassel wrote:
> > Add a helper function to convert a size to the representation used by the
> > Resizable BAR Capability Register.
> 
> > +/**
> > + * pci_epc_bar_size_to_rebar_cap() - convert a size to the representation used
> > + *				     by the Resizable BAR Capability Register
> > + * @size: the size to convert
> > + * @cap: where to store the result
> > + *
> > + * Returns 0 on success and a negative error code in case of error.
> > + */
> > +int pci_epc_bar_size_to_rebar_cap(size_t size, u32 *cap)
> > +{
> > +	/*
> > +	 * According to PCIe base spec, min size for a resizable BAR is 1 MB,
> > +	 * thus disallow a requested BAR size smaller than 1 MB.
> > +	 * Disallow a requested BAR size larger than 128 TB.
> > +	 */
> > +	if (size < SZ_1M || (u64)size > (SZ_128G * 1024))
> > +		return -EINVAL;
> > +
> > +	*cap = ilog2(size) - ilog2(SZ_1M);
> > +
> > +	/* Sizes in REBAR_CAP start at BIT(4). */
> > +	*cap = BIT(*cap + 4);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(pci_epc_bar_size_to_rebar_cap);
> 
> This doesn't seem specific to EPC.  It looks basically similar to
> pci_rebar_bytes_to_size().  Can we consolidate anything there?

This function is to convert a size to
"Resizable BAR Capability Register", which includes one or more supported
BAR sizes.

This register should only ever by written by a PCI endpoint function,
so I think its current home in drivers/pci/endpoint/ is correct.


pci_rebar_bytes_to_size() is used to convert a size to
"Resizable BAR Control Register", specifically the field
"BAR Size".

This "BAR Size" field in the "Resizable BAR Control Register" can be
written by the host side (or endpoint side), to select one of the
supported bar sizes. So I think it makes sense for
pci_rebar_bytes_to_size() to live in pci.h.


> 
> Since we're basing this on the spec, we should include a complete
> citation, e.g., PCIe r6.0, sec xxx.

I agree:
https://lore.kernel.org/linux-pci/20250219171454.2903059-2-cassel@kernel.org/T/#u


Kind regards,
Niklas