linux-kernel - Re: [PATCH v2 0/2] PCI: xgene: Restore working PCIe functionnality

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Date:   Mon, 21 Mar 2022 09:50:24 -0600
From:   dann frazier <dann.frazier@...onical.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        PCI <linux-pci@...r.kernel.org>,
        Toan Le <toan@...amperecomputing.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 0/2] PCI: xgene: Restore working PCIe functionnality

On Mon, Mar 21, 2022 at 10:17:34AM -0500, Rob Herring wrote:
> On Mon, Mar 21, 2022 at 5:49 AM Marc Zyngier <maz@...nel.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). 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.

Happy to provide that for the m400 if told how :)

  -dann

> 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.