Re: [RFC PATCH] PCI: Remove End-End TLP as PASID dependency

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Sinan]

On Wed, Jun 10, 2020 at 12:18:14PM +0800, Zhangfei Gao wrote:
> Some platform devices appear as PCI and have PCI cfg space,
> but are actually on the AMBA bus.
> They can support PASID via smmu stall feature, but does not
> support tlp since they are not real pci devices.
> So remove tlp as a PASID dependency.

When you iterate on this, pay attention to things like:

  - Wrap paragraphs to 75 columns or so, so they fill the whole line
    but don't overflow when "git show" adds 4 spaces.

  - Leave a blank line between paragraphs.

  - Capitalize consistently: "SMMU", "PCI", "TLP".

  - Provide references to relevant spec sections, e.g., for the SMMU
    stall feature.

> Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> ---
>  drivers/pci/ats.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 390e92f..8e31278 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -344,9 +344,6 @@ int pci_enable_pasid(struct pci_dev *pdev, int features)
>  	if (WARN_ON(pdev->pasid_enabled))
>  		return -EBUSY;
>  
> -	if (!pdev->eetlp_prefix_path)
> -		return -EINVAL;

No.  This would mean we might enable PASID on actual PCIe devices when
it is not safe to do so, as Jean-Philippe pointed out.

You cannot break actual PCIe devices just to make your non-PCIe device
work.

These devices do not support PASID as defined in the PCIe spec.  They
might support something *like* PASID, and you might be able to make
parts of the PCI core work with them, but you're going to have to deal
with the parts that don't follow the PCIe spec on your own.  That
might be quirks, it might be some sort of AMBA adaptation shim, I
don't know.  But it's not the responsibility of the PCI core to adapt
to them.

>  	if (!pasid)
>  		return -EINVAL;
>  
> -- 
> 2.7.4
>