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

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Message-ID: <50d9910b-dbd4-48d1-ad43-f298d14986fe@amd.com>
Date: Thu, 5 Sep 2024 09:46:44 -0500
From: Wei Huang <wei.huang2@....com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, netdev@...r.kernel.org,
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Subject: Re: [PATCH V4 03/12] PCI/TPH: Add pcie_tph_modes() to query TPH modes



On 9/4/24 14:40, Bjorn Helgaas wrote:
> 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.

I can get rid of pcie_tph_modes() if unnecessary.

The design logic was that a driver can be used on various devices from
the same company. Some of these devices might not be TPH capable. So
instead of using trial-and-error (i.e. try INT_VEC ==> DEV_SPEC ==> give
up), we provide a way for the driver to query the device TPH
capabilities and pick a mode explicitly. IMO the code will be a bit cleaner.

> 
> 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.

In the real world cases I saw, this is true. In the spec I didn't find
that these bits are mutually exclusive though.

> 
> Bjorn