Re: [PATCH v2 05/11] PCI/ATS: Gather checks into pci_ats_supported()

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Mar 11, 2020 at 01:45:00PM +0100, Jean-Philippe Brucker wrote:
> IOMMU drivers need to perform several tests when checking if a device
> supports ATS.  Move them all into a new function that returns true when
> a device and its host bridge support ATS.
> 
> Since pci_enable_ats() now calls pci_ats_supported(), the following
> new checks are now common:
> * whether a device is trusted.  Devices plugged into external-facing
>   ports such as thunderbolt are untrusted.
> * whether the host bridge supports ATS, which defaults to true unless
>   the firmware description states that ATS isn't supported by the host
>   bridge.
> 
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
>  drivers/pci/ats.c       | 30 +++++++++++++++++++++++++++++-
>  include/linux/pci-ats.h |  3 +++
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 390e92f2d8d1..bbfd0d42b8b9 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -30,6 +30,34 @@ void pci_ats_init(struct pci_dev *dev)
>  	dev->ats_cap = pos;
>  }
>  
> +/**
> + * pci_ats_supported - check if the device can use ATS
> + * @dev: the PCI device
> + *
> + * Returns true if the device supports ATS and is allowed to use it, false
> + * otherwise.
> + */
> +bool pci_ats_supported(struct pci_dev *dev)
> +{
> +	struct pci_host_bridge *bridge;
> +
> +	if (!dev->ats_cap)
> +		return false;
> +
> +	if (dev->untrusted)
> +		return false;
> +
> +	bridge = pci_find_host_bridge(dev->bus);
> +	if (!bridge)
> +		return false;
> +
> +	if (!bridge->ats_supported)
> +		return false;
> +
> +	return true;

I assume this is the same as

  return bridge->ats_supported;

Only "assuming" because I'm not a C language lawyer, but I assume it
does the obvious conversion from unsigned:1 to bool.

> +}