Re: [PATCH] nvme-pci: Disable LTR for simple suspend

[Christoph Hellwig <hch@lst.de>]

On Mon, Mar 14, 2022 at 08:55:37AM -0500, Mario Limonciello wrote:
> Some drives from SSSTC are showing stability problems after s0i3
> entry when the Linux kernel is in s2idle loop if LTR has been
> enabled. This leads to failures to resume.
> 
> This appears to be a firmware issue specific to SSSTC SSDs, but to
> avoid this class of problem, disable LTR when going into s2idle and
> simple suspend has been set.

This seems like a giant hammer to do this for all NVMe devices,
why not quirk the specific ones?

> +static void nvme_suspend_ltr(struct device *dev, bool disable)
> +{
> +	struct pci_dev *pdev = to_pci_dev(dev);
> +	struct nvme_dev *ndev = pci_get_drvdata(pdev);
> +
> +	if (disable) {
> +		u16 word;
> +
> +		pcie_capability_read_word(pdev, PCI_EXP_DEVCTL2, &word);
> +		ndev->restore_ltr = word & PCI_EXP_DEVCTL2_LTR_EN;
> +		pcie_capability_clear_word(pdev, PCI_EXP_DEVCTL2,
> +					   PCI_EXP_DEVCTL2_LTR_EN);
> +	} else if (ndev->restore_ltr) {
> +		pcie_capability_set_word(pdev, PCI_EXP_DEVCTL2,
> +					 PCI_EXP_DEVCTL2_LTR_EN);
> +	}
> +}

The calling conventions of this function are rather strange by
mixing up two very different things.

I think two PCI-level helpers to disable LTR and return the status
it ways in and to enable LTR would be really nice to have here.

>  	if (ndev->last_ps == U32_MAX ||
>  	    nvme_set_power_state(ctrl, ndev->last_ps) != 0)
>  		goto reset;
> @@ -3239,6 +3259,11 @@ static int nvme_suspend(struct device *dev)
>  
>  	ndev->last_ps = U32_MAX;
>  
> +	/* If using s2idle with simple suspend, disable LTR to avoid problems. */

Overly long line here.