linux-kernel - Re: [PATCH] PCI: tegra: Use PCI_CONF1_EXT

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Date:   Mon, 17 Oct 2022 09:40:53 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc:     Pali Rohár <pali@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Vidya Sagar <vidyas@...dia.com>, linux-tegra@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: tegra: Use PCI_CONF1_EXT_ADDRESS() macro


On 17/10/2022 08:43, Lorenzo Pieralisi wrote:
> On Tue, Oct 11, 2022 at 05:47:50PM +0100, Jon Hunter wrote:
>>
>> On 11/10/2022 17:16, Pali Rohár wrote:
>>
>> ...
>>
>>> I see, this is stupid mistake. PCIe config read and write operations
>>> needs to be 4-byte aligned, so normally it is done by calculating 4-byte
>>> aligned base address and then using appropriate cpu load/store
>>> instruction to access just defined size/offset of 4-byte config space
>>> register.
>>>
>>> pci-tegra.c is using common helper functions pci_generic_config_read()
>>> and pci_generic_config_write(), which expects final address with offset,
>>> and not 4-byte aligned address.
>>>
>>> I'm not sure what should be the proper fix, but for me it looks like
>>> that pci_generic_config_read() and pci_generic_config_write() could be
>>> adjusted to handle it.
>>>
>>> In any case, above patch is a regressions and I see there two options
>>> for now:
>>>
>>> 1) Reverting that patch
>>>
>>> 2) Adding "offset |= where & 0x3;" after the PCI_CONF1_EXT_ADDRESS()
>>>      macro to set also lower 2 bits of accessed register.
>>>
>>> Jon, Lorenzo, what do you think? Could you test if 2) is working fine?
>>
>>
>> I tested 'offset |= where & 0xff' which is essentially the same as the above
>> and that is working and so I am sure that the above works too. However, I do
>> wonder if reverting is simpler because we already have a '&
>> ~PCI_CONF1_ENABLE' and now adding '| where & 0x3' seems to diminish the
>> value of this change.
> 
> Hi Jon,
> 
> it is unfortunate but I think we should proceed with a revert, please
> send it and I shall ask Bjorn to send it for one of the upcoming -rcX.

I have sent a revert for this change.

Thanks
Jon

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