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    SubjectRe: [PATCH v2 0/2] PCI: xgene: Restore working PCIe functionnality
    On Mon, Mar 21, 2022 at 01:03:27PM -0500, Rob Herring wrote:
    > On Mon, Mar 21, 2022 at 11:36 AM Marc Zyngier <maz@kernel.org> wrote:
    > >
    > > On Mon, 21 Mar 2022 15:17:34 +0000,
    > > Rob Herring <robh@kernel.org> wrote:
    > > >
    > > > On Mon, Mar 21, 2022 at 5:49 AM Marc Zyngier <maz@kernel.org> wrote:
    > > > >
    > > > > Since 6dce5aa59e0b ("PCI: xgene: Use inbound resources for setup") was
    > > > > merged in the 5.5 time frame, PCIe on the venerable XGene platform has
    > > > > been unusable: 6dce5aa59e0b broke both XGene-1 (Mustang and m400) and
    > > > > XGene-2 (Merlin), while the addition of c7a75d07827a ("PCI: xgene: Fix
    > > > > IB window setup") fixed XGene-2, but left the rest of the zoo
    > > > > unusable.
    > > > >
    > > > > It is understood that this systems come with "creative" DTs that don't
    > > > > match the expectations of modern kernels. However, there is little to
    > > > > be gained by forcing these changes on users -- the firmware is not
    > > > > upgradable, and the current owner of the IP will deny that these
    > > > > machines have ever existed.
    > > >
    > > > The gain for fixing this properly is not having drivers do their own
    > > > dma-ranges parsing. We've seen what happens when drivers do their own
    > > > parsing of standard properties (e.g. interrupt-map).
    > >
    > > We have, and we added the required exceptions for the legacy platforms
    > > that the code base supported until then. We didn't leave things broken
    > > just because we didn't like the way things were done a long time ago.
    > >
    > > > Currently, we don't have any drivers doing their own parsing:
    > > >
    > > > $ git grep of_pci_dma_range_parser_init
    > > > drivers/of/address.c:int of_pci_dma_range_parser_init(struct
    > > > of_pci_range_parser *parser,
    > > > drivers/of/address.c:EXPORT_SYMBOL_GPL(of_pci_dma_range_parser_init);
    > > > drivers/of/address.c:#define of_dma_range_parser_init
    > > > of_pci_dma_range_parser_init
    > > > drivers/of/unittest.c: if (of_pci_dma_range_parser_init(&parser, np)) {
    > > > drivers/pci/of.c: err = of_pci_dma_range_parser_init(&parser, dev_node);
    > > > include/linux/of_address.h:extern int
    > > > of_pci_dma_range_parser_init(struct of_pci_range_parser *parser,
    > > > include/linux/of_address.h:static inline int
    > > > of_pci_dma_range_parser_init(struct of_pci_range_parser *parser,
    > > >
    > > > And we can probably further refactor this to be private to drivers/pci/of.c.
    > > >
    > > > For XGene-2 the issue is simply that the driver depends on the order
    > > > of dma-ranges entries.
    > > >
    > > > For XGene-1, I'd still like to understand what the issue is. Reverting
    > > > the first fix and fixing 'dma-ranges' should have fixed it. I need a
    > > > dump of how the IB registers are initialized in both cases. I'm not
    > > > saying changing 'dma-ranges' in the firmware is going to be required
    > > > here. There's a couple of other ways we could fix that without a
    > > > firmware change, but first I need to understand why it broke.
    > >
    > > Reverting 6dce5aa59e0b was enough for me, without changing anything
    > > else.
    >
    > Meaning c7a75d07827a didn't matter for you. I'm not sure that it would.
    >
    > Can you tell me what 'dma-ranges' contains on your system?
    >
    > > m400 probably uses an even older firmware (AFAIR, it was stuck
    > > with an ancient version of u-boot that HP never updated, while Mustang
    > > had a few updates). In any case, that DT cannot be changed.
    >
    > How is Dann changing it then? I assume he's not changing the firmware,
    > but overriding it. That could be a possible solution.

    Correct, I'm just overriding it for testing. I'm using the pxelinux
    emulation provided by the m400's u-boot, which supports an FDT field:

    ---------
    $ cat /srv/tftp/pxelinux.cfg/default
    DEFAULT default

    LABEL default
    KERNEL uImage
    APPEND initrd=uInitrd console=ttyS0,9600n8r ro root=LABEL=cloudimg-rootfs
    FDT m400.dtb
    ---------

    This loads the specified file into ${fdt_addr_r}, overriding the blob
    that the firmware had already loaded there.

    > Do the DT's in the kernel tree correspond to anything anyone is
    > using?

    Upstream apm-mustang.dtb is what Ubuntu uses for Mustang boards w/
    u-boot firmware. That used to work fine, but I haven't tried lately.

    -dann

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