Re: [PATCH V6 09/13] pci, acpi: Support for ACPI based generic PCI host controller

From: Tomasz Nowicki
Date: Mon May 02 2016 - 07:32:05 EST

On 04/29/2016 07:35 PM, Jayachandran C wrote:
On Fri, Apr 29, 2016 at 2:07 PM, Lorenzo Pieralisi
<lorenzo.pieralisi@xxxxxxx> wrote:
On Thu, Apr 28, 2016 at 04:48:00PM -0500, Bjorn Helgaas wrote:


+static int pci_acpi_setup_ecam_mapping(struct acpi_pci_root *root,
+ struct acpi_pci_generic_root_info *ri)
+ u16 seg = root->segment;
+ u8 bus_start = root->secondary.start;
+ u8 bus_end = root->secondary.end;
+ struct pci_config_window *cfg;
+ struct mcfg_entry *e;
+ phys_addr_t addr;
+ int err = 0;
+ mutex_lock(&pci_mcfg_lock);
What does this lock protect? The pci_mcfg_list should already be
initialized by the time we get there, and it should be immutable for
the life of the system. In fact, I would prefer if we could just
search the static table itself whenever we need it rather than caching
it in our own list. But I don't think we can easily do that because
acpi_table_parse() is __init.

+ e = pci_mcfg_lookup(seg, bus_start);
I would argue that we should check for _CBA first, and fall back to
MCFG if _CBA doesn't exist.

+ if (!e) {
+ addr = acpi_pci_root_get_mcfg_addr(root->device->handle);
IMO, acpi_pci_root_get_mcfg_addr() is misnamed. It should be
acpi_pci_config_base_addr() or similar. It definitely is not related
to MCFG. Not your fault, obviously.

+ if (addr == 0) {
+ pr_err(PREFIX"%04x:%02x-%02x bus range error\n",
+ seg, bus_start, bus_end);
+ err = -ENOENT;
+ goto err_out;
+ }
+ } else {
+ if (bus_start != e->bus_start) {
+ pr_err("%04x:%02x-%02x bus range mismatch %02x\n",
+ seg, bus_start, bus_end, e->bus_start);
+ err = -EINVAL;
+ goto err_out;
+ } else if (bus_end != e->bus_end) {
+ pr_warn("%04x:%02x-%02x bus end mismatch %02x\n",
+ seg, bus_start, bus_end, e->bus_end);
+ bus_end = min(bus_end, e->bus_end);
+ }
+ addr = e->addr;
+ }
I really don't think you need a lock around this, so you can factor
out the address lookup into something like:

addr = acpi_pci_config_base_addr(...);
if (addr)
return addr;

return acpi_pci_mcfg_lookup(seg, busn_res);

You can check inside acpi_pci_mcfg_lookup() to make sure the entry you
find covers the entire [busn_res.start-busn_res.end] range and return
failure if it doesn't. At this point, I'm not sure it's worth it to
truncate the host bridge bus range to match something we find in MCFG.

If the MCFG entry covers *more* than the host bridge range from _CRS,
that's fine. In any case, we have to be careful with the start address,
because the MCFG start address is always based on bus 0, but I think
pci_generic_ecam_create() expects the start address based on the
bus_start you pass to it.
Yes, I spotted this too, it is unfortunate but DT and MCFG handle
the ECAM regions differently. In DT the reg property is relative
to bus_start - ie reg MMIO region maps config space starting at
the first bus in bus-range:


in ACPI(MCFG) as you said it is always relative to bus 0, it is
unfortunate but the address to be mapped should be computed
differently in the ECAM layer.
Can't this be handled by fixing up the address before passing to

I agree, this should work, IMO.