linux-kernel - Re: [PATCH v2 04/15] PCI: Add pci_find_vsec

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Date:   Tue, 2 Feb 2021 19:08:55 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        dmaengine@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 04/15] PCI: Add pci_find_vsec_capability() to find a
 specific VSEC

On Tue, Feb 02, 2021 at 01:40:18PM +0100, Gustavo Pimentel wrote:
>  /**
> + * pci_find_vsec_capability - Find a vendor-specific extended capability
> + * @dev: PCI device to query
> + * @cap: vendor-specific capability ID code
> + *
> + * Returns the address of the vendor-specific structure that matches the
> + * requested capability ID code within the device's PCI configuration space
> + * or 0 if it does not find a match.
> + */
> +u16 pci_find_vsec_capability(struct pci_dev *dev, int vsec_cap_id)
> +{

As the name implies, the capability is "vendor-specific", so it is perfectly
possible that two vendors use the same VSEC ID for different things.

To make sure you're looking for the right capability, you need to pass
a u16 vendor into this function and bail out if dev->vendor is different.


> +	u16 vsec;
> +	u32 header;
> +
> +	vsec = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_VNDR);
> +	while (vsec) {
> +		if (pci_read_config_dword(dev, vsec + PCI_VSEC_HDR,
> +					  &header) == PCIBIOS_SUCCESSFUL &&
> +		    PCI_VSEC_CAP_ID(header) == vsec_cap_id)
> +			return vsec;
> +
> +		vsec = pci_find_next_ext_capability(dev, vsec,
> +						    PCI_EXT_CAP_ID_VNDR);
> +	}

FWIW, a more succinct implementation would be:

	while ((vsec = pci_find_next_ext_capability(...))) { ... }

See set_pcie_thunderbolt() in drivers/pci/probe.c for an example.
Please consider refactoring that function to use your new helper.

Thanks,

Lukas