Re: [PATCH for-linus v2 3/3] PCI/bwctrl: Enable only if more than one speed is supported

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Sun, 15 Dec 2024 11:20:53 +0100
Lukas Wunner <lukas@wunner.de> wrote:

> If a PCIe port only supports a single speed, enabling bandwidth control
> is pointless:  There's no need to monitor autonomous speed changes, nor
> can the speed be changed.
> 
> Not enabling it saves a small amount of memory and compute resources,
> but also fixes a boot hang reported by Niklas:  It occurs when enabling
> bandwidth control on Downstream Ports of Intel JHL7540 "Titan Ridge 2018"
> Thunderbolt controllers.  The ports only support 2.5 GT/s in accordance
> with USB4 v2 sec 11.2.1, so the present commit works around the issue.
> 
> PCIe r6.2 sec 8.2.1 prescribes that:
> 
>    "A device must support 2.5 GT/s and is not permitted to skip support
>     for any data rates between 2.5 GT/s and the highest supported rate."
> 
> Consequently, bandwidth control is currently only disabled if a port
> doesn't support higher speeds than 2.5 GT/s.  However the Implementation
> Note in PCIe r6.2 sec 7.5.3.18 cautions:
> 
>    "It is strongly encouraged that software primarily utilize the
>     Supported Link Speeds Vector instead of the Max Link Speed field,
>     so that software can determine the exact set of supported speeds on
>     current and future hardware.  This can avoid software being confused
>     if a future specification defines Links that do not require support
>     for all slower speeds."
> 
> In other words, future revisions of the PCIe Base Spec may allow gaps
> in the Supported Link Speeds Vector.  To be future-proof, don't just
> check whether speeds above 2.5 GT/s are supported, but rather check
> whether *more than one* speed is supported.

It has long felt like the need for the very low speeds will become optional
eventually, though the challenges of getting a backwards compatibility
change into the specification may make that take a while.  Hence
I agree this approach makes sense.

Reviewed-by: Jonathan Cameron <Jonthan.Cameron@huawei.com>

> 
> Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
> Reported-by: Niklas Schnelle <niks@kernel.org>
> Closes: https://lore.kernel.org/r/db8e457fcd155436449b035e8791a8241b0df400.camel@kernel.org/
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> ---
>  drivers/pci/pcie/portdrv.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pcie/portdrv.c b/drivers/pci/pcie/portdrv.c
> index 5e10306b6308..02e73099bad0 100644
> --- a/drivers/pci/pcie/portdrv.c
> +++ b/drivers/pci/pcie/portdrv.c
> @@ -265,12 +265,14 @@ static int get_port_device_capability(struct pci_dev *dev)
>  	    (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
>  		services |= PCIE_PORT_SERVICE_DPC;
>  
> +	/* Enable bandwidth control if more than one speed is supported. */
>  	if (pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM ||
>  	    pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
>  		u32 linkcap;
>  
>  		pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &linkcap);
> -		if (linkcap & PCI_EXP_LNKCAP_LBNC)
> +		if (linkcap & PCI_EXP_LNKCAP_LBNC &&
> +		    hweight8(dev->supported_speeds) > 1)
>  			services |= PCIE_PORT_SERVICE_BWCTRL;
>  	}
>