Re: [PATCH INTERNAL 3/3] PCI: iproc: Disable MSI parsing in certain PAXC blocks

[Lorenzo Pieralisi]

On Tue, May 22, 2018 at 09:48:55AM -0700, Ray Jui wrote:
> Hi Robin/Lorenzo,
> 
> On 5/21/2018 7:26 AM, Robin Murphy wrote:
> >On 21/05/18 14:33, Lorenzo Pieralisi wrote:
> >>[+Robin]
> >>
> >>On Fri, May 18, 2018 at 12:48:28PM -0700, Ray Jui wrote:
> >>>Hi Lorenzo,
> >>>
> >>>On 5/18/2018 6:56 AM, Lorenzo Pieralisi wrote:
> >>>>On Fri, May 18, 2018 at 02:53:41PM +0530, poza@xxxxxxxxxxxxxx wrote:
> >>>>>On 2018-05-17 22:51, Ray Jui wrote:
> >>>>>>The internal MSI parsing logic in certain revisions of PAXC root
> >>>>>>complexes does not work properly and can casue corruptions on the
> >>>>>>writes. They need to be disabled
> >>>>>>
> >>>>>>Signed-off-by: Ray Jui <ray.jui@xxxxxxxxxxxx>
> >>>>>>Reviewed-by: Scott Branden <scott.branden@xxxxxxxxxxxx>
> >>>>>>---
> >>>>>>drivers/pci/host/pcie-iproc.c | 34 ++++++++++++++++++++++++++++++++--
> >>>>>>1 file changed, 32 insertions(+), 2 deletions(-)
> >>>>>>
> >>>>>>diff --git a/drivers/pci/host/pcie-iproc.c
> >>>>>>b/drivers/pci/host/pcie-iproc.c
> >>>>>>index 3c76c5f..b906d80 100644
> >>>>>>--- a/drivers/pci/host/pcie-iproc.c
> >>>>>>+++ b/drivers/pci/host/pcie-iproc.c
> >>>>>>@@ -1144,10 +1144,22 @@ static int
> >>>>>>iproc_pcie_paxb_v2_msi_steer(struct
> >>>>>>iproc_pcie *pcie, u64 msi_addr)
> >>>>>>    return ret;
> >>>>>>}
> >>>>>>
> >>>>>>-static void iproc_pcie_paxc_v2_msi_steer(struct
> >>>>>>iproc_pcie *pcie, u64
> >>>>>>msi_addr)
> >>>>>>+static void iproc_pcie_paxc_v2_msi_steer(struct
> >>>>>>iproc_pcie *pcie, u64
> >>>>>>msi_addr,
> >>>>>>+                     bool enable)
> >>>>>>{
> >>>>>>    u32 val;
> >>>>>>
> >>>>>>+    if (!enable) {
> >>>>>>+        /*
> >>>>>>+         * Disable PAXC MSI steering. All write transfers will be
> >>>>>>+         * treated as non-MSI transfers
> >>>>>>+         */
> >>>>>>+        val = iproc_pcie_read_reg(pcie, IPROC_PCIE_MSI_EN_CFG);
> >>>>>>+        val &= ~MSI_ENABLE_CFG;
> >>>>>>+        iproc_pcie_write_reg(pcie, IPROC_PCIE_MSI_EN_CFG, val);
> >>>>>>+        return;
> >>>>>>+    }
> >>>>>>+
> >>>>>>    /*
> >>>>>>     * Program bits [43:13] of address of
> >>>>>>GITS_TRANSLATER register into
> >>>>>>     * bits [30:0] of the MSI base address register.  In
> >>>>>>fact, in all iProc
> >>>>>>@@ -1201,7 +1213,7 @@ static int
> >>>>>>iproc_pcie_msi_steer(struct iproc_pcie
> >>>>>>*pcie,
> >>>>>>            return ret;
> >>>>>>        break;
> >>>>>>    case IPROC_PCIE_PAXC_V2:
> >>>>>>-        iproc_pcie_paxc_v2_msi_steer(pcie, msi_addr);
> >>>>>>+        iproc_pcie_paxc_v2_msi_steer(pcie, msi_addr, true);
> >>>>>
> >>>>>Are you calling iproc_pcie_paxc_v2_msi_steer() anywhere
> >>>>>else with 'false' ?
> >>>>>I see its getting called only from one place in current code
> >>>>>iproc_pcie_msi_steer().
> >>>>
> >>>>It is called below with the false field to disable MSIs.
> >>>>That's probably
> >>>>one reason more to create a function to enable/disable MSIs instead of
> >>>>adding a parameter to iproc_pcie_paxc_v2_msi_steer().
> >>>
> >>>Correct. Thanks for helping to explain.
> >>>
> >>>>
> >>>>Which brings me to the question: what happens to those MSIs writes
> >>>>when MSIs are disabled according to this patch ? Are they terminated
> >>>>at the root complex ?
> >>>
> >>>Note the PAXC block parses MSI writes from our internally connected
> >>>endpoints (i.e., an embedded network processor). The PAXC block examines
> >>>these MSI writes and modifies the memory attributes (to DEVICE) of these
> >>>data and then send them out to the AXI bus. These MSI writes
> >>>will then be
> >>>forwarded to the GIC (e.g., GICv2m, GICv3-ITS from ARM) for further
> >>>processing. This is saying, PAXC itself does not process these
> >>>MSI writes.
> >>>They are processed by the GIC and associated interrupt will be generated
> >>>form the GIC. PAXC's job is to parse and tag them properly so these MSI
> >>>writes can reach the GIC, and at the same, reach the GIC at
> >>>the right order.
> >>>
> >>>On some of these problematic PAXCs, we are being forced to
> >>>disable this PAXC
> >>>internal parsing logic. In this case, we set up static mapping with the
> >>>IOMMU to modify the memory attributes of these MSI writes.
> >>>These MSI writes
> >>>are always designated towards a specific memory address (e.g.,
> >>>on the GICv3
> >>>based system, it's the address of the translator register),
> >>>and that's why
> >>>static mapping table can be set up to work around this.
> >>
> >>Which means, I presume, that there must be some code that somehow
> >>somewhere in the kernel sets-up those mappings and it has to be related
> >>to this patch, in which case I wonder why we enable the PAXC steering at
> >>all given that this (hack) can be done through the IOMMU (and I assume
> >>that on all PAXC platforms that do need MSIs there is an IOMMU IP
> >>upstream the root bridge, otherwise I have no idea what will happen to
> >>those MSI writes).
> >
> >If it's only rewriting memory attributes (FWIW it sounds like this
> >thing is comparable to the AXI translation table of the PLDA root
> >complex we have in Juno), then if the IOMMU is controlled by Linux
> >the PAXC shouldn't be needed anyway. In that situation the
> >GICv2m/ITS doorbell will be already mapped for the endpoint as
> >device memory (and usually at some arbitrary address that the PAXC
> >won't recognise anyway) - see iommu_dma_get_msi_page().
> >
> >If Linux *doesn't* know about the IOMMU, then the firmware should
> >be free to set it up with a static 1:1 mapping of the PA space and
> >leave it that way, provided Linux can't inadvertently stomp on
> >those page tables later.
> 
> Right, in our case, we had our ATF bootloader set up 1:1 mapping for
> this. In this case, PAXC internal MSI parsing is completely disabled
> (which is what this patch does for those PAXC blocks that have this
> issue).

Ok so to sum it up, why does the PCI host bridge code has to deal with
this steering at all given the discussion above ? Why can't we disable
PAXC steering completely ?

Lorenzo