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

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Message-ID: <20241028165031.GA1106195@bhelgaas>
Date: Mon, 28 Oct 2024 11:50:31 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Michał Winiarski <michal.winiarski@...el.com>
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 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".

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:

> + * 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);