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

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Message-ID: <20200610074633.GA6844@myrica>
Date:   Wed, 10 Jun 2020 09:46:33 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Zhangfei Gao <zhangfei.gao@...aro.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, kenneth-lee-2012@...mail.com,
        Wangzhou <wangzhou1@...ilicon.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] PCI: Remove End-End TLP as PASID dependency

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.
> 
> Signed-off-by: Zhangfei Gao <zhangfei.gao@...aro.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;
> -

This check is useful, and follows the PCI specification (4.0r1.0
2.2.10.2 End-End TLP Prefix Processing: "Software should ensure that TLPs
containing End-End TLP Prefixes are not sent to components that do not
support them.")

Why not set the eetlp_prefix_path bit from a PCI quirk?  Unlike the stall
problem from the other thread, this one looks like a simple design mistake
that can be fixed easily in future iterations of the platform: just set
the "End-End TLP Prefix Supported" bit in the Device Capability 2 Register
of all bridges.

Thanks,
Jean