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

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Message-ID: <18a48407-9fd7-175d-0c7f-5702b077f13d@nvidia.com>
Date:   Tue, 11 Oct 2022 17:47:50 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        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 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.

Cheers
Jon

-- 
nvpublic