netdev - Re: [PATCH V4 03/12] PCI/TPH: Add pcie_tph

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Message-ID: <20240904194052.GA344429@bhelgaas>
Date: Wed, 4 Sep 2024 14:40:52 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Wei Huang <wei.huang2@....com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, netdev@...r.kernel.org,
	Jonathan.Cameron@...wei.com, corbet@....net, davem@...emloft.net,
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	manoj.panicker2@....com, Eric.VanTassell@....com,
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	bhelgaas@...gle.com, lukas@...ner.de, paul.e.luse@...el.com,
	jing2.liu@...el.com
Subject: Re: [PATCH V4 03/12] PCI/TPH: Add pcie_tph_modes() to query TPH modes

On Thu, Aug 22, 2024 at 03:41:11PM -0500, Wei Huang wrote:
> Add pcie_tph_modes() to allow drivers to query the TPH modes supported
> by an endpoint device, as reported in the TPH Requester Capability
> register. The modes are reported as a bitmask and current supported
> modes include:
> 
>  - PCI_TPH_CAP_NO_ST: NO ST Mode Supported
>  - PCI_TPH_CAP_INT_VEC: Interrupt Vector Mode Supported
>  - PCI_TPH_CAP_DEV_SPEC: Device Specific Mode Supported

> + * pcie_tph_modes - Get the ST modes supported by device
> + * @pdev: PCI device
> + *
> + * Returns a bitmask with all TPH modes supported by a device as shown in the
> + * TPH capability register. Current supported modes include:
> + *   PCI_TPH_CAP_NO_ST - NO ST Mode Supported
> + *   PCI_TPH_CAP_INT_VEC - Interrupt Vector Mode Supported
> + *   PCI_TPH_CAP_DEV_SPEC - Device Specific Mode Supported
> + *
> + * Return: 0 when TPH is not supported, otherwise bitmask of supported modes
> + */
> +int pcie_tph_modes(struct pci_dev *pdev)
> +{
> +	if (!pdev->tph_cap)
> +		return 0;
> +
> +	return get_st_modes(pdev);
> +}
> +EXPORT_SYMBOL(pcie_tph_modes);

I'm not sure I see the need for pcie_tph_modes().  The new bnxt code
looks like this:

  bnxt_request_irq
    if (pcie_tph_modes(bp->pdev) & PCI_TPH_CAP_INT_VEC)
      rc = pcie_enable_tph(bp->pdev, PCI_TPH_CAP_INT_VEC);

What is the advantage of this over just this?

  bnxt_request_irq
    rc = pcie_enable_tph(bp->pdev, PCI_TPH_CAP_INT_VEC);

It seems like drivers could just ask for what they want since
pcie_enable_tph() has to verify support for it anyway.  If that fails,
the driver can fall back to another mode.

Returning a bitmask of supported modes might be useful if the driver
could combine them, but IIUC the modes are all mutually exclusive, so
the driver can't request a combination of them.

Bjorn