Re: [PATCH v4] iommu/of: Fix pci_request_acs() before enumerating PCI devices

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc John, who tested 6bf6c24720d3]

On Fri, May 21, 2021 at 03:03:24AM +0000, Wang Xingang wrote:
> From: Xingang Wang <wangxingang5@huawei.com>
> 
> When booting with devicetree, the pci_request_acs() is called after the
> enumeration and initialization of PCI devices, thus the ACS is not
> enabled. And ACS should be enabled when IOMMU is detected for the
> PCI host bridge, so add check for IOMMU before probe of PCI host and call
> pci_request_acs() to make sure ACS will be enabled when enumerating PCI
> devices.

I'm happy to apply this, but I'm a little puzzled about 6bf6c24720d3
("iommu/of: Request ACS from the PCI core when configuring IOMMU
linkage").  It was tested and fixed a problem, but I don't understand
how.

6bf6c24720d3 added the call to pci_request_acs() in
of_iommu_configure() so it currently looks like this:

  of_iommu_configure(dev, ...)
  {
    if (dev_is_pci(dev))
      pci_request_acs();

pci_request_acs() sets pci_acs_enable, which tells us to enable ACS
when enumerating PCI devices in the future.  But we only call
pci_request_acs() if we already *have* a PCI device.

So maybe 6bf6c24720d3 fixed a problem for *some* PCI devices, but not
all?  E.g., did we call of_iommu_configure() for one PCI device before
enumerating the rest?

> Fixes: 6bf6c24720d33 ("iommu/of: Request ACS from the PCI core when
> configuring IOMMU linkage")
> Signed-off-by: Xingang Wang <wangxingang5@huawei.com>
> ---
>  drivers/iommu/of_iommu.c | 1 -
>  drivers/pci/of.c         | 8 +++++++-
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> index a9d2df001149..54a14da242cc 100644
> --- a/drivers/iommu/of_iommu.c
> +++ b/drivers/iommu/of_iommu.c
> @@ -205,7 +205,6 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  			.np = master_np,
>  		};
>  
> -		pci_request_acs();
>  		err = pci_for_each_dma_alias(to_pci_dev(dev),
>  					     of_pci_iommu_init, &info);
>  	} else {
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index da5b414d585a..2313c3f848b0 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -581,9 +581,15 @@ static int pci_parse_request_of_pci_ranges(struct device *dev,
>  
>  int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge)
>  {
> -	if (!dev->of_node)
> +	struct device_node *node = dev->of_node;
> +
> +	if (!node)
>  		return 0;
>  
> +	/* Detect IOMMU and make sure ACS will be enabled */
> +	if (of_property_read_bool(node, "iommu-map"))
> +		pci_request_acs();
> +
>  	bridge->swizzle_irq = pci_common_swizzle;
>  	bridge->map_irq = of_irq_parse_and_map_pci;
>  
> -- 
> 2.19.1
>