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

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Date:   Tue, 22 Mar 2022 14:56:29 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>
Cc:     dann frazier <dann.frazier@...onical.com>,
        "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 2022-03-22 14:39, Rob Herring wrote:
> On Tue, Mar 22, 2022 at 01:16:35PM +0000, Robin Murphy wrote:
>> On 2022-03-21 20:06, Robin Murphy wrote:
>>> On 2022-03-21 19:21, Marc Zyngier wrote:
>>>> On Mon, 21 Mar 2022 18:03:27 +0000,
>>>> Rob Herring <robh@...nel.org> wrote:
>>>>>
>>>>> On Mon, Mar 21, 2022 at 11:36 AM Marc Zyngier <maz@...nel.org> wrote:
>>>>>>
>>>>>> On Mon, 21 Mar 2022 15:17:34 +0000,
>>>>>> Rob Herring <robh@...nel.org> wrote:
>>>>>>>
>>>>>>> On Mon, Mar 21, 2022 at 5:49 AM Marc Zyngier <maz@...nel.org> wrote:
>>>>>>>>
>>>>>>> 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?
>>>>
>>>> Each pcie node (all 5 of them) has:
>>>>
>>>> dma-ranges = <0x42000000 0x80 0x00 0x80 0x00 0x00 0x80000000
>>>> ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ 0x42000000 0x00 0x00 0x00 0x00 0x80 0x00>;
> 
> This is the same as what Stï¿½phane has for Merlin. So c7a75d07827a ("PCI:
> xgene: Fix IB window setup") should have fixed Mustang.

Unless XGene 1 has some weird implicit requirement on the order in which 
the registers are programmed, that XGene 2 doesn't. And from looking at 
the code, I don't see any obvious less-mad possibility to explain the 
breakage.

>>> Hmm, is there anyone other than iommu-dma who actually depends on the
>>> resource list being sorted in ascending order of bus address? I recall
>>> at the time I pushed for creating the list in sorted order as it was the
>>> simplest and most efficient option, but there's no technical reason we
>>> couldn't create it in as-found order and defer the sorting until
>>> iova_reserve_pci_windows() (at worst that could even operate on a
>>> temporary copy if need be). It's just more code, which didn't need to
>>> exist without a good reason, but if this is one then exist it certainly
>>> may.
>>
>> Taking a closer look, the Cadence driver is already re-sorting the list
>> for its own setup, so iommu-dma can't assume the initial sort is
>> preserved and needs to do its own anyway. Does the (untested) diff below
>> end up helping X-Gene also?
> 
> There's no IOMMU on X-Gene 1 or 2 based on the upstream dts files, so
> how would this matter?

Because devm_of_pci_get_host_bridge_resources() is forcing the 
dma_ranges list to be in a different order from the original DT for 
iommu-dma's benefit, but whether iommu-dma actually consumes it or not 
later is immaterial.

Robin.