netdev - Re: [RESEND PATCH V3 6/6] PCI: Enable 10-Bit tag support for PCIe RP devices

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Date:   Mon, 14 Jun 2021 06:57:36 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Dongdong Liu <liudongdong3@...wei.com>
Cc:     helgaas@...nel.org, hch@...radead.org, kw@...ux.com,
        linux-pci@...r.kernel.org, rajur@...lsio.com,
        hverkuil-cisco@...all.nl, linux-media@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [RESEND PATCH V3 6/6] PCI: Enable 10-Bit tag support for PCIe RP
 devices

On Sun, Jun 13, 2021 at 05:29:15PM +0800, Dongdong Liu wrote:
> PCIe spec 5.0r1.0 section 2.2.6.2 implementation note, In configurations
> where a Requester with 10-Bit Tag Requester capability needs to target
> multiple Completers, one needs to ensure that the Requester sends 10-Bit
> Tag Requests only to Completers that have 10-Bit Tag Completer capability.
> So we enable 10-Bit Tag Requester for root port only when the devices
> under the root port support 10-Bit Tag Completer.
> 
> Signed-off-by: Dongdong Liu <liudongdong3@...wei.com>
> ---
>  drivers/pci/pcie/portdrv_pci.c | 75 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
> 
> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
> index c7ff1ee..baf413f 100644
> --- a/drivers/pci/pcie/portdrv_pci.c
> +++ b/drivers/pci/pcie/portdrv_pci.c
> @@ -90,6 +90,78 @@ static const struct dev_pm_ops pcie_portdrv_pm_ops = {
>  #define PCIE_PORTDRV_PM_OPS	NULL
>  #endif /* !PM */
>  
> +static int pci_10bit_tag_comp_support(struct pci_dev *dev, void *data)
> +{
> +	u8 *support = data;
> +
> +	if (*support == 0)
> +		return 0;
> +
> +	if (!pci_is_pcie(dev)) {
> +		*support = 0;
> +		return 0;
> +	}
> +
> +	/*
> +	 * PCIe spec 5.0r1.0 section 2.2.6.2 implementation note.
> +	 * For configurations where a Requester with 10-Bit Tag Requester
> +	 * capability targets Completers where some do and some do not have
> +	 * 10-Bit Tag Completer capability, how the Requester determines which
> +	 * NPRs include 10-Bit Tags is outside the scope of this specification.
> +	 * So we do not consider hotplug scenario.
> +	 */
> +	if (dev->is_hotplug_bridge) {
> +		*support = 0;
> +		return 0;
> +	}
> +
> +	if (!(dev->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_COMP)) {
> +		*support = 0;
> +		return 0;
> +	}
> +
> +	return 0;
> +}
> +
> +static void pci_configure_rp_10bit_tag(struct pci_dev *dev)
> +{
> +	u8 support = 1;
> +	struct pci_dev *pchild;
> +
> +	if (dev->subordinate == NULL)
> +		return;
> +
> +	/* If no devices under the root port, no need to enable 10-Bit Tag. */
> +	pchild = list_first_entry_or_null(&dev->subordinate->devices,
> +					  struct pci_dev, bus_list);
> +	if (pchild == NULL)
> +		return;

pchild is never used after this check, so this could be simplified to
a list_empty(&dev->subordinate->devices).