Re: [PATCH v2 1/7] ACPI/IORT: Check ATS capability in root complex nodes

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Apr 09, 2019 at 05:52:39PM +0100, Jean-Philippe Brucker wrote:
> Root complex node in IORT has a bit telling whether it supports ATS or
> not. Store this bit in the IOMMU fwspec when setting up a device, so it
> can be accessed later by an IOMMU driver.
> 
> Use the negative version (NO_ATS) at the moment because it's not clear
> if/how the bit needs to be integrated in other firmware descriptions. The
> SMMU has a feature bit telling if it supports ATS, which might be
> sufficient in most systems for deciding whether or not we should enable
> the ATS capability in endpoints.

Hmm, the SMMUv3 architecture manual is pretty explicit about this:

  | It [SMMU_IDR0.ATS] does not guarantee that client devices and intermediate
  | components also support ATS and this must be determined separately.

so we may need to extend the PCI bindings to describe this. I think the
negative logic is likely to get in the way if that's the case.

> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker-5wv7dgnIgG8@public.gmane.org>
> ---
>  drivers/acpi/arm64/iort.c | 11 +++++++++++
>  include/linux/iommu.h     |  4 ++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> index e48894e002ba..7f2c1c9c6b38 100644
> --- a/drivers/acpi/arm64/iort.c
> +++ b/drivers/acpi/arm64/iort.c
> @@ -1028,6 +1028,14 @@ void iort_dma_setup(struct device *dev, u64 *dma_addr, u64 *dma_size)
>  	dev_dbg(dev, "dma_pfn_offset(%#08llx)\n", offset);
>  }
>  
> +static bool iort_pci_rc_supports_ats(struct acpi_iort_node *node)
> +{
> +	struct acpi_iort_root_complex *pci_rc;
> +
> +	pci_rc = (struct acpi_iort_root_complex *)node->node_data;
> +	return pci_rc->ats_attribute & ACPI_IORT_ATS_SUPPORTED;
> +}

Do we need to worry about the "noats" command-line option here? It feels
like we should be checking with the PCI subsystem before telling the SMMU
we're good to go.

I'll need Lorenzo's ack on this.

Will