linux-kernel - Re: [PATCH] PCI: j721e: Fix the value of linkdown_irq

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Message-ID: <20250314041705.v5j2fjulol5ywvyq@uda0492258>
Date: Fri, 14 Mar 2025 09:47:05 +0530
From: Siddharth Vadapalli <s-vadapalli@...com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: Siddharth Vadapalli <s-vadapalli@...com>, <lpieralisi@...nel.org>,
        <kw@...ux.com>, <vigneshr@...com>, <manivannan.sadhasivam@...aro.org>,
        <robh@...nel.org>, <bhelgaas@...gle.com>, <rogerq@...nel.org>,
        <linux-omap@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <srk@...com>
Subject: Re: [PATCH] PCI: j721e: Fix the value of linkdown_irq_regfield for
 J784S4

On Thu, Mar 13, 2025 at 11:02:15AM -0500, Bjorn Helgaas wrote:
> On Thu, Mar 13, 2025 at 11:25:19AM +0530, Siddharth Vadapalli wrote:
> > On Wed, Mar 12, 2025 at 11:16:00AM -0500, Bjorn Helgaas wrote:
> > > On Wed, Mar 05, 2025 at 06:50:18PM +0530, Siddharth Vadapalli wrote:
> > > > Commit under Fixes assigned the value of 'linkdown_irq_regfield' for the
> > > > J784S4 SoC as 'LINK_DOWN' which corresponds to BIT(1). However, according
> > > > to the Technical Reference Manual and Register Documentation for the J784S4
> > > > SoC [0], BIT(1) corresponds to "ENABLE_SYS_EN_PCIE_DPA_1" which is __NOT__
> > > > the field for the link-state interrupt. Instead, it is BIT(10) of the
> > > > "PCIE_INTD_ENABLE_REG_SYS_2" register that corresponds to the link-state
> > > > field named as "ENABLE_SYS_EN_PCIE_LINK_STATE".
> > > 
> > > I guess the reason we want this is that on J784S4, we ignore actual
> > > link-down interrupts (and we don't write STATUS_CLR_REG_SYS_2 to clear
> > > the interrupt indication, so maybe there's an interrupt storm), and we
> > > think some other interrupt (DPA_1, whatever that is) is actually a
> > > link-down interrupt?
> > 
> > While it is true that actual link-down interrupts are ignored, it is not
> > the case that there's an interrupt storm because the same incorrect macro
> > is used to enable the interrupt line. Since the enables an interrupt for
> > DPA_1 which never fires, we don't run into the situation where we are not
> > clearing the interrupt (the interrupt handler will look for the same
> > incorrect field to clear the interrupt if it does fire for DPA_1, but that
> > doesn't happen). The 'linkdown_irq_regfield' corresponds to the
> > "link-state" field not just in the J784S4 SoC, but in all SoCs supported by
> > the pci-j721e.c driver. It is only in J721E that it is BIT(1)
> > [LINK_DOWN macro], while in all other SoCs (J784S4 included), it is BIT(10)
> > [J7200_LINK_DOWN macro since it was first added for J7200 SoC]. Matt
> > probably referred to J721E's Technical Reference Manual and ended up
> > incorrectly assigning "LINK_DOWN", due to which the driver is enabling
> > the DPA_1 interrupt and the interrupt handler is also going to look for
> > the field corresponding to receiving an interrupt for DPA_1.
> 
> So I guess without this patch, we incorrectly ignore link-down
> interrupts on J784S4.  It's good to have a one-sentence motivation
> like that somewhere in the commit log that we can pull out and include
> in the merge commit log and the pull request.

Yes, we can prepend the following to the existing commit message:
"Link down interrupts on J784S4 SoC are missed because..."

resulting in the following updated paragraph in the commit message:
Link down interrupts on J784S4 SoC are missed because commit under Fixes
assigned the value of 'linkdown_irq_regfield' for the....


> 
> > I can only hope that the URL will redirect to the latest version of
> > the User Guide if at all it becomes invalid.
> 
> OK, thanks, I guess there's nothing more to do ;)  I guess that manual
> is not really designed for collaborative development.
> 
> Thanks for the patient hand holding!

:)

Regards,
Siddharth.