Re: [PATCH] PCI: j721e: Fix the value of linkdown_irq_regfield for J784S4

[Bjorn Helgaas <helgaas@kernel.org>]

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.

> 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!

Bjorn