linux-kernel - Re: [PATCH v5 03/14] PCI: cadence: Convert all r/w accessors to perform only 32-bit accesses

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Date:   Mon, 25 May 2020 09:00:36 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Rob Herring <robh@...nel.org>
CC:     Tom Joseph <tjoseph@...ence.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <devicetree@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 03/14] PCI: cadence: Convert all r/w accessors to
 perform only 32-bit accesses

Hi Rob,

On 5/22/2020 9:24 PM, Rob Herring wrote:
> On Thu, May 21, 2020 at 9:37 PM Kishon Vijay Abraham I <kishon@...com> wrote:
>>
>> Certain platforms like TI's J721E using Cadence PCIe IP can perform only
>> 32-bit accesses for reading or writing to Cadence registers. Convert all
>> read and write accesses to 32-bit in Cadence PCIe driver in preparation
>> for adding PCIe support in TI's J721E SoC.
> 
> Looking more closely I don't think cdns_pcie_ep_assert_intx is okay
> with this and never can be given the PCI_COMMAND and PCI_STATUS
> registers are in the same word (IIRC, that's the main reason 32-bit
> config space accesses are broken). So this isn't going to work at

right, PCI_STATUS has write '1' to clear bits and there's a chance that it
could be reset while raising legacy interrupt. While this cannot be avoided for
TI's J721E, other platforms doesn't have to have this limitation.
> least for EP accesses. And maybe you need a custom .raise_irq() hook
> to minimize any problems (such as making the RMW atomic at least from
> the endpoint's perspective).

This is to make sure EP doesn't update in-consistent state when RC is updating
the PCI_STATUS register? Since this involves two different systems, how do we
make this atomic?

Thanks
Kishon