linux-kernel - Re: [PATCH v4] PCI: dwc: pci-dra7xx: Fix MSI IRQ handling

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Date:   Mon, 30 Mar 2020 11:37:03 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Vignesh Raghavendra <vigneshr@...com>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        linux-omap@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] PCI: dwc: pci-dra7xx: Fix MSI IRQ handling

On Mon, Mar 30, 2020 at 11:29:52AM -0500, Bjorn Helgaas wrote:
> [+cc Marc, Thomas]
> 
> On Fri, Mar 27, 2020 at 03:24:34PM +0530, Vignesh Raghavendra wrote:
> > Due an issue with PCIe wrapper around DWC PCIe IP on dra7xx, driver
> > needs to ensure that there are no pending MSI IRQ vector set (i.e
> > PCIE_MSI_INTR0_STATUS reads 0 at least once) before exiting IRQ handler.
> > Else, the dra7xx PCIe wrapper will not register new MSI IRQs even though
> > PCIE_MSI_INTR0_STATUS shows IRQs are pending.
> 
> I'm not an IRQ guy (real IRQ guys CC'd), but I'm wondering if this is
> really a symptom of a problem in the generic DWC IRQ handling, not a
> problem in dra7xx itself.
> 
> I thought it was sort of standard behavior that a device would not
> send a new MSI unless there was a transition from "no status bits set"
> to "at least one status bit set".  I'm looking at this text from the
> PCIe r5.0 spec, sec 6.7.3.4:
> 
>   If the Port is enabled for edge-triggered interrupt signaling using
>   MSI or MSI-X, an interrupt message must be sent every time the
>   logical AND of the following conditions transitions from FALSE to
>   TRUE:
> 
>     - The associated vector is unmasked (not applicable if MSI does
>       not support PVM).
> 
>     - The Hot-Plug Interrupt Enable bit in the Slot Control register
>       is set to 1b.
> 
>     - At least one hot-plug event status bit in the Slot Status
>       register and its associated enable bit in the Slot Control
>       register are both set to 1b.
> 
> and this related commit: https://git.kernel.org/linus/fad214b0aa72

and this one: https://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git/commit/?id=87d94ad41bd2