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

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Message-ID: <457db3ae-e68a-d2fc-ba5f-5393ad464413@ti.com>
Date:   Thu, 28 May 2020 03:36:14 +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 5/27/2020 10:07 PM, Rob Herring wrote:
> On Wed, May 27, 2020 at 4:49 AM Kishon Vijay Abraham I <kishon@...com> wrote:
>>
>> 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).
> 
> But you don't need a lock everywhere. You need locks in the callers
> (and only sometimes).

Okay, the locks should be added only for registers where HOST can also write to
the same register? Maybe only raise_irq then..

> 
>> 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.
> 
> Now I'm wondering how set_msi is not racy in the current code with the
> host setting/clearing PCI_MSI_FLAGS_ENABLE? Maybe that bit is RO from
> the EP side?

set_msi/set_msix is a one time configuration that is invoked before the host
establishes the link with the endpoint. I don't think we have to consider this
as racy.

Thanks
Kishon

> 
> Ultimately I think you're going to have to provide your own endpoint
> functions or you need accessors for specific registers like
> PCI_MSI_FLAGS. Then for example, you just rely on the 2 bytes before
> PCI_MSI_FLAGS being reserved and do a 32-bit access without a RMW.
> Trying to abstract this at the register read/write level is going to
> be fragile
> 
> Rob
>