linux-kernel - Re: [PATCH v2 2/3] PCI: j721e: Add PCI legacy interrupt support for J721E

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Date:   Tue, 10 Aug 2021 13:33:21 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Tom Joseph <tjoseph@...ence.com>, <linux-omap@...r.kernel.org>,
        <linux-pci@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Lokesh Vutla <lokeshvutla@...com>
Subject: Re: [PATCH v2 2/3] PCI: j721e: Add PCI legacy interrupt support for J721E

On Mon, 09 Aug 2021 15:58:38 +0100,
Kishon Vijay Abraham I <kishon@...com> wrote:
> 
> Hi Marc,
> 
> On 09/08/21 3:09 pm, Marc Zyngier wrote:
> > On Mon, 09 Aug 2021 05:50:10 +0100,
> > Kishon Vijay Abraham I <kishon@...com> wrote:
> >>
> >> Hi Marc,
> >>
> >> On 04/08/21 8:43 pm, Marc Zyngier wrote:
> >>> On Wed, 04 Aug 2021 14:29:11 +0100,
> >>> Kishon Vijay Abraham I <kishon@...com> wrote:
> >>>>
> >>>> Add PCI legacy interrupt support for J721E. J721E has a single HW
> >>>> interrupt line for all the four legacy interrupts INTA/INTB/INTC/INTD.
> >>>> The HW interrupt line connected to GIC is a pulse interrupt whereas
> >>>> the legacy interrupts by definition is level interrupt. In order to
> >>>> provide level interrupt functionality to edge interrupt line, PCIe
> >>>> in J721E has provided IRQ_EOI register.
> >>>>
> >>>> However due to Errata ID #i2094 ([1]), EOI feature is not enabled in HW
> >>>> and only a single pulse interrupt will be generated for every
> >>>> ASSERT_INTx/DEASSERT_INTx.
> >>>
> >>> So my earlier remark stands. If you get a single edge, how do you
> >>> handle a level that is still high after having handled the interrupt
> >>> on hardware that has this bug?
> >>
> >> Right, this hardware (J721E) has a bug but was fixed in J7200 (Patch 3/3
> >> handles that).
> > 
> > But how do you make it work with J721E? Is it even worth supporting if
> > (as I expect) it is unreliable?
> 
> I've seen at-least the NVMe devices triggers the interrupts again and
> the data transfer completes. But I agree, this is unreliable.

Then I don't think you should add INTx support for this system. It is
bound to be a support burden, and will reflect badly on the whole
platform. Focusing on J7200 is probably the best thing to do.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.