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 10:37:44 -0600
From:   Rob Herring <robh@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
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

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).

> 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?

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