linux-kernel - Re: [PATCH v4 6/7] PCI: Allow drivers to control VF BAR size

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Message-ID: <f5z3pmw7jdxfeej56imz3i5s7upakjj2m6hyadtgpasvrvzciq@76cfywmwqkt5>
Date: Tue, 12 Nov 2024 15:55:58 +0100
From: Michał Winiarski <michal.winiarski@...el.com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: <linux-pci@...r.kernel.org>, <intel-xe@...ts.freedesktop.org>,
	<dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>, "Bjorn
 Helgaas" <bhelgaas@...gle.com>, Christian König
	<christian.koenig@....com>, Krzysztof Wilczyński
	<kw@...ux.com>, Ilpo Järvinen
	<ilpo.jarvinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Michal Wajdeczko <michal.wajdeczko@...el.com>, Lucas De Marchi
	<lucas.demarchi@...el.com>, Thomas Hellström
	<thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>, Matt Roper <matthew.d.roper@...el.com>
Subject: Re: [PATCH v4 6/7] PCI: Allow drivers to control VF BAR size

On Mon, Oct 28, 2024 at 11:50:31AM -0500, Bjorn Helgaas wrote:
> On Fri, Oct 25, 2024 at 11:50:37PM +0200, Michał Winiarski wrote:
> > Drivers could leverage the fact that the VF BAR MMIO reservation is
> > created for total number of VFs supported by the device by resizing the
> > BAR to larger size when smaller number of VFs is enabled.
> > 
> > Add a pci_iov_vf_bar_set_size() function to control the size and a
> > pci_iov_vf_bar_get_sizes() helper to get the VF BAR sizes that will
> > allow up to num_vfs to be successfully enabled with the current
> > underlying reservation size.
> > ...
> 
> > + * pci_iov_vf_bar_get_sizes - get VF BAR sizes that allow to create up to num_vfs
> > + * @dev: the PCI device
> > + * @resno: the resource number
> > + * @num_vfs: number of VFs
> > + *
> > + * Get the sizes of a VF resizable BAR that can fit up to num_vfs within the
> > + * resource that reserves the MMIO space (originally up to total_VFs) the as
> > + * bitmask defined in the spec (bit 0=1MB, bit 19=512GB).
> 
> This sentence doesn't quite parse; something is missing around "the as".

Yeah, typo, "the" should be removed.

> I'm guessing you mean to say something about the return value being a
> bitmask of VF BAR sizes that can be accommodated if num_vfs are
> enabled?  If so, maybe combine it with the following paragraph:

I'll change it to:

"Get the sizes of a VF resizable BAR that can be accomodated within the
resource that reserves the MMIO space if num_vfs are enabled.

Returns 0 if BAR isn't resizable, otherwise returns a bitmask in format
defined in the spec (bit 0=1MB, bit 19=512GB)."

-Michał

> 
> > + * Returns 0 if BAR isn't resizable.
> > + *
> > + */
> > +u32 pci_iov_vf_bar_get_sizes(struct pci_dev *dev, int resno, int num_vfs)
> > +{
> > +	resource_size_t size;
> > +	u32 sizes;
> > +	int i;
> > +
> > +	sizes = pci_rebar_get_possible_sizes(dev, resno);
> > +	if (!sizes)
> > +		return 0;
> > +
> > +	while (sizes > 0) {
> > +		i = __fls(sizes);
> > +		size = pci_rebar_size_to_bytes(i);
> > +
> > +		if (size * num_vfs <= pci_resource_len(dev, resno))
> > +			break;
> > +
> > +		sizes &= ~BIT(i);
> > +	}
> > +
> > +	return sizes;
> > +}
> > +EXPORT_SYMBOL_GPL(pci_iov_vf_bar_get_sizes);