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:   Wed, 27 May 2020 16:19:31 +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 5/26/2020 8:42 PM, Rob Herring wrote:
> On Sun, May 24, 2020 at 9:30 PM Kishon Vijay Abraham I <kishon@...com> wrote:
>>
>> 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?
> 
> You can't make it atomic WRT both systems, but is there locking around
> each RMW? Specifically, are preemption and interrupts disabled to
> ensure time between a read and write are minimized? You wouldn't want
> interrupts disabled during the delay too though (i.e. around
> .raise_irq()).

Okay, I'll add spin spin_lock_irqsave() in cdns_pcie_write_sz(). As you also
pointed below that delay for legacy interrupt is wrong and it has to be fixed
(with a later series).

How do you want to handle cdns_pcie_ep_fn_writew() now? Because now we are
changing the default implementation to perform only 32-bit access (used for
legacy interrupt, msi-x interrupt and while writing standard headers) and it's
not okay only for legacy interrupts for platforms other than TI.

So just for legacy interrupt, you want me to add a different accessor which
does not perform 32-bit writes (while we add a different .raise_irq for TI
platform?
> 
> BTW, I've asked this question before, but aren't PCI legacy interrupts
> level triggered? If so, isn't generating a pulse wrong?

You are right. This is wrong and it has to be fixed. I'll work on this later.

Thanks
Kishon