[PATCH v2 9/9] PCI: endpoint: Set prefetch when allocating memory for 64-bit BARs

Niklas Cassel
2024-03-12T10:52:51Z

From the PCIe 6.0 base spec:
"Generally only 64-bit BARs are good candidates, since only Legacy
Endpoints are permitted to set the Prefetchable bit in 32-bit BARs,
and most scalable platforms map all 32-bit Memory BARs into
non-prefetchable Memory Space regardless of the Prefetchable bit value."

"For a PCI Express Endpoint, 64-bit addressing must be supported for all
BARs that have the Prefetchable bit Set. 32-bit addressing is permitted
for all BARs that do not have the Prefetchable bit Set."

"Any device that has a range that behaves like normal memory should mark
the range as prefetchable. A linear frame buffer in a graphics device is
an example of a range that should be marked prefetchable."

The PCIe spec tells us that we should have the prefetchable bit set for
64-bit BARs backed by "normal memory". The backing memory that we allocate
for a 64-bit BAR using pci_epf_alloc_space() (which calls
dma_alloc_coherent()) is obviously "normal memory".

Thus, set the prefetchable bit when allocating backing memory for a 64-bit
BAR.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
drivers/pci/endpoint/pci-epf-core.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/drivers/pci/endpoint/pci-epf-core.c b/drivers/pci/endpoint/pci-epf-core.c
index e7dbbeb1f0de..20d2bde0747c 100644
--- a/drivers/pci/endpoint/pci-epf-core.c
+++ b/drivers/pci/endpoint/pci-epf-core.c
@@ -309,6 +309,9 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
else
epf_bar[bar].flags |= PCI_BASE_ADDRESS_MEM_TYPE_32;

+ if (epf_bar[bar].flags & PCI_BASE_ADDRESS_MEM_TYPE_64)
+ epf_bar[bar].flags |= PCI_BASE_ADDRESS_MEM_PREFETCH;
+
return space;
}
EXPORT_SYMBOL_GPL(pci_epf_alloc_space);
--
2.44.0