On Wed, Feb 28, 2024 at 09:02:11AM -0600, Bjorn Helgaas wrote: > On Wed, Feb 28, 2024 at 06:34:11PM +0530, Mrinmay Sarkar wrote: > > On 2/24/2024 4:24 AM, Bjorn Helgaas wrote: > > > On Fri, Feb 23, 2024 at 07:33:38PM +0530, Mrinmay Sarkar wrote: > > > > Due to some hardware changes, SA8775P has set the NO_SNOOP attribute > > > > in its TLP for all the PCIe controllers. NO_SNOOP attribute when set, > > > > the requester is indicating that there no cache coherency issues exit > > > > for the addressed memory on the host i.e., memory is not cached. But > > > > in reality, requester cannot assume this unless there is a complete > > > > control/visibility over the addressed memory on the host. > > > > > > Forgive my ignorance here. It sounds like the cache coherency issue > > > would refer to system memory, so the relevant No Snoop attribute would > > > be in DMA transactions, i.e., Memory Reads or Writes initiated by PCIe > > > Endpoints. But it looks like this patch would affect TLPs initiated > > > by the Root Complex, not those from Endpoints, so I'm confused about > > > how this works. > > > > > > If this were in the qcom-ep driver, it would make sense that setting > > > No Snoop in the TLPs initiated by the Endpoint could be a problem, but > > > that doesn't seem to be what this patch is concerned with. > > > > I think in multiprocessor system cache coherency issue might occur. > > and RC as well needs to snoop cache to avoid coherency as it is not > > enable by default. > > My mental picture isn't detailed enough, so I'm still confused. We're > talking about TLPs initiated by the RC. Normally these would be > because a driver did a CPU load or store to a PCIe device MMIO space, > not to system memory. > Endpoint can expose its system memory as a BAR to the host. In that case, the cache coherency issue would apply for TLPs originating from RC as well. - Mani > But I guess you're suggesting the RC can initiate a TLP with a system > memory address? And this TLP would be routed not to a Root Port or to > downstream devices, but it would instead be kind of a loopback and be > routed back up through the RC and maybe IOMMU, to system memory? > > I would have expected accesses like this to be routed directly to > system memory without ever reaching the PCIe RC. > > > and we are enabling this feature for qcom-ep driver as well. > > it is in patch2. > > > > Thanks > > Mrinmay > > > > > > And worst case, if the memory is cached on the host, it may lead to > > > > memory corruption issues. It should be noted that the caching of memory > > > > on the host is not solely dependent on the NO_SNOOP attribute in TLP. > > > > > > > > So to avoid the corruption, this patch overrides the NO_SNOOP attribute > > > > by setting the PCIE_PARF_NO_SNOOP_OVERIDE register. This patch is not > > > > needed for other upstream supported platforms since they do not set > > > > NO_SNOOP attribute by default. > > > > > > > > 8775 has IP version 1.34.0 so intruduce a new cfg(cfg_1_34_0) for this > > > > platform. Assign enable_cache_snoop flag into struct qcom_pcie_cfg and > > > > set it true in cfg_1_34_0 and enable cache snooping if this particular > > > > flag is true. > > > s/intruduce/introduce/ > > > > > > Bjorn -- மணிவண்ணன் சதாசிவம்
