+ Arnd On Wed, Mar 13, 2024 at 11:58:01AM +0100, Niklas Cassel wrote: > From the PCIe 6.0 base spec: It'd be good to mention the section also. > "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". > I'm not sure this is correct. Memory returned by 'dma_alloc_coherent' is not the 'normal memory' but rather 'consistent/coherent memory'. Here the question is, can the memory returned by dma_alloc_coherent() be prefetched or write-combined on all architectures. I hope Arnd can answer this question. - Mani > Thus, set the prefetchable bit when allocating backing memory for a 64-bit > BAR. > > Signed-off-by: Niklas Cassel <cassel@xxxxxxxxxx> > --- > 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 > -- மணிவண்ணன் சதாசிவம்
