Re: [PATCH for-linus v2 1/3] PCI: Assume 2.5 GT/s if Max Link Speed is undefined

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

On Sun, 15 Dec 2024 11:20:51 +0100
Lukas Wunner <lukas@wunner.de> wrote:

> Broken PCIe devices may not set any of the bits in the Link Capabilities
> Register's "Max Link Speed" field.  Assume 2.5 GT/s in such a case,
> which is the lowest possible PCIe speed.  It must be supported by every
> device per PCIe r6.2 sec 8.2.1.
> 
> Emit a message informing about the malformed field.  Use KERN_INFO
> severity to minimize annoyance.  This will help silicon validation
> engineers take note of the issue so that regular users hopefully never
> see it.
> 
> There is currently no known affected product, but a subsequent commit
> will honor the Max Link Speed field when determining supported speeds
> and depends on the field being well-formed.  (It uses the Max Link Speed
> as highest bit in a GENMASK(highest, lowest) macro and if the field is
> zero, that would result in GENMASK(0, lowest).)
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
Seems like this is the best we can do for this (hopefully)
theoretical hardware bug.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
>  drivers/pci/pci.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 35dc9f249b86..ab0ef7b6c798 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -6233,6 +6233,13 @@ u8 pcie_get_supported_speeds(struct pci_dev *dev)
>  	u32 lnkcap2, lnkcap;
>  	u8 speeds;
>  
> +	/* A device must support 2.5 GT/s (PCIe r6.2 sec 8.2.1) */
> +	pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &lnkcap);
> +	if (!(lnkcap & PCI_EXP_LNKCAP_SLS)) {
> +		pci_info(dev, "Undefined Max Link Speed; assume 2.5 GT/s\n");
> +		return PCI_EXP_LNKCAP2_SLS_2_5GB;
> +	}
> +
>  	/*
>  	 * Speeds retain the reserved 0 at LSB before PCIe Supported Link
>  	 * Speeds Vector to allow using SLS Vector bit defines directly.
> @@ -6244,8 +6251,6 @@ u8 pcie_get_supported_speeds(struct pci_dev *dev)
>  	if (speeds)
>  		return speeds;
>  
> -	pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &lnkcap);
> -
>  	/* Synthesize from the Max Link Speed field */
>  	if ((lnkcap & PCI_EXP_LNKCAP_SLS) == PCI_EXP_LNKCAP_SLS_5_0GB)
>  		speeds = PCI_EXP_LNKCAP2_SLS_5_0GB | PCI_EXP_LNKCAP2_SLS_2_5GB;