Re: [PATCH 2/4] x86/amd_nb: add support for newer PCI topologies

Bjorn Helgaas
2018-11-02T19:59:25Z

On Fri, Nov 02, 2018 at 06:11:07PM +0000, Woods, Brian wrote:
> Add support for new processors which have multiple PCI root complexes
> per data fabric/SMN interface. The interfaces per root complex are
> redundant and should be skipped. This makes sure the DF/SMN interfaces
> get accessed via the correct root complex.

SMN?

> Ex:
> DF/SMN 0 -> 60
> 40
> 20
> 00
> DF/SMN 1 -> e0
> c0
> a0
> 80

This isn't my code, and I'm not really objecting to these changes, but
from where I sit, the fact that you need this sort of vendor-specific
topology discovery is a little bit ugly and seems like something of a
maintenance issue. You could argue that this is sort of an "AMD CPU
driver", which is entitled to be device-specific, and that does make
some sense.

But device-specific code is typically packaged as a driver that uses
driver registration interfaces like acpi_bus_register_driver(),
pci_register_driver(), etc. That gives you a consistent structure
and, more importantly, a framework for dealing with hotplug. It
doesn't look like amd_nb.c would deal well with hot-add of CPUs.

> Signed-off-by: Brian Woods <brian.woods@amd.com>
> ---
> arch/x86/kernel/amd_nb.c | 41 +++++++++++++++++++++++++++++++++++------
> 1 file changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/amd_nb.c b/arch/x86/kernel/amd_nb.c
> index 19d489ee2b1e..c0bf26aeb7c3 100644
> --- a/arch/x86/kernel/amd_nb.c
> +++ b/arch/x86/kernel/amd_nb.c
> @@ -213,7 +213,10 @@ int amd_cache_northbridges(void)
> const struct pci_device_id *root_ids = amd_root_ids;
> struct pci_dev *root, *misc, *link;
> struct amd_northbridge *nb;
> - u16 i = 0;
> + u16 roots_per_misc = 0;
> + u16 misc_count = 0;
> + u16 root_count = 0;
> + u16 i, j;
>
> if (amd_northbridges.num)
> return 0;
> @@ -226,26 +229,52 @@ int amd_cache_northbridges(void)
>
> misc = NULL;
> while ((misc = next_northbridge(misc, misc_ids)) != NULL)
> - i++;
> + misc_count++;
>
> - if (!i)
> + root = NULL;
> + while ((root = next_northbridge(root, root_ids)) != NULL)
> + root_count++;
> +
> + if (!misc_count)
> return -ENODEV;
>
> - nb = kcalloc(i, sizeof(struct amd_northbridge), GFP_KERNEL);
> + if (root_count) {
> + roots_per_misc = root_count / misc_count;
> +
> + /*
> + * There should be _exactly_ N roots for each DF/SMN
> + * interface.
> + */
> + if (!roots_per_misc || (root_count % roots_per_misc)) {
> + pr_info("Unsupported AMD DF/PCI configuration found\n");
> + return -ENODEV;
> + }
> + }
> +
> + nb = kcalloc(misc_count, sizeof(struct amd_northbridge), GFP_KERNEL);
> if (!nb)
> return -ENOMEM;
>
> amd_northbridges.nb = nb;
> - amd_northbridges.num = i;
> + amd_northbridges.num = misc_count;
>
> link = misc = root = NULL;
> - for (i = 0; i != amd_northbridges.num; i++) {
> + for (i = 0; i < amd_northbridges.num; i++) {
> node_to_amd_nb(i)->root = root =
> next_northbridge(root, root_ids);
> node_to_amd_nb(i)->misc = misc =
> next_northbridge(misc, misc_ids);
> node_to_amd_nb(i)->link = link =
> next_northbridge(link, link_ids);
> +
> + /*
> + * If there are more root devices than data fabric/SMN,
> + * interfaces, then the root devices per DF/SMN
> + * interface are redundant and N-1 should be skipped so
> + * they aren't mapped incorrectly.
> + */
> + for (j = 1; j < roots_per_misc; j++)
> + root = next_northbridge(root, root_ids);
> }
>
> if (amd_gart_present())
> --
> 2.11.0
>