Re: [PATCH v5 1/3] PCI: qcom: Enable cache coherency for SA8775P RC

[Bjorn Helgaas <helgaas@kernel.org>]

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.

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