linux-kernel - Re: [PATCH] PCI: xilinx: Remove platform/architecture restrictions

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Message-ID: <83428319.6ITjLHsrBP@np-p-burton>
Date:   Mon, 31 Jul 2017 16:49:59 -0700
From:   Paul Burton <paul.burton@...tec.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Guenter Roeck <linux@...ck-us.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Michal Simek <michal.simek@...inx.com>,
        Sören Brinkmann <soren.brinkmann@...inx.com>,
        James Hogan <james.hogan@...tec.com>
Subject: Re: [PATCH] PCI: xilinx: Remove platform/architecture restrictions

On Monday, 31 July 2017 16:36:08 PDT Bjorn Helgaas wrote:
> On Mon, Jul 31, 2017 at 04:19:13PM -0700, Paul Burton wrote:
> > Hi Bjorn,
> > 
> > On Monday, 31 July 2017 15:58:22 PDT Bjorn Helgaas wrote:
> > > On Mon, Jul 24, 2017 at 11:49:22AM +0100, Paul Burton wrote:
> > > > Hi Guenter & all,
> > > > 
> > > > On Monday, 24 July 2017 01:39:37 BST Guenter Roeck wrote:
> > > > > The MIPS Boston board configuration tries to enable
> > > > > CONFIG_PCIE_XILINX.
> > > > > That doesn't work since PCIE_XILINX depends on ARCH_ZYNQ ||
> > > > > MICROBLAZE.
> > > > > Remove that restriction.
> > > > 
> > > > I'd prefer that this patch does not go in standalone. The intent for
> > > > the
> > > > MIPS Boston board is that this driver is enabled for MIPS by this
> > > > patch:
> > > > 
> > > > https://patchwork.kernel.org/patch/9794361/
> > > > 
> > > > But not until after earlier patches in that series fix issues with the
> > > > driver:
> > > > 
> > > > https://patchwork.kernel.org/patch/9794355/
> > > > https://patchwork.kernel.org/patch/9794357/
> > > > https://patchwork.kernel.org/patch/9794359/
> > > > 
> > > > That has been held up by disagreement about whether the driver should
> > > > be
> > > > using 0-3 or 1-4 for hardware IRQ numbers, sadly, despite the driver
> > > > already being in tree & clearly broken, and my series not changing
> > > > which
> > > > the driver uses...
> > > 
> > > It's true that your v5 series only changes xilinx from using hwirq 0-3
> > > to 0-4 (with 0 being unused in both cases, and the addition of 4
> > > fixing the "INTD doesn't work" bug).
> > 
> > That isn't true - the xilinx-pcie driver already uses 1-4, and my change
> > simply prevents it from hitting a WARN() in the IRQ code when doing so.
> 
> My apologies.  I was relying on the changelog, which says the current
> code "creates an IRQ domain of size 4 (ie.  IRQ numbers 0 through 3)"
> and the patch:
> 
>   -       port->leg_domain = irq_domain_add_linear(pcie_intc_node, 4,
>   +       port->leg_domain = irq_domain_add_linear(pcie_intc_node, 1 + 4,
> 
> I'm not enough of an IRQ expert to understand why what I said was
> incorrect (other than maybe INTD actually works, but emits a warning?)

The driver does create an IRQ domain of size 4, as though it is going to use 
numbering 0-3 with it. However the driver then goes on to use numbers 1-4, 
which leads to a warning from the IRQ code because the domain isn't big enough 
to cover the case where hwirq=4 (ie. INTD).

It still works because irq_domain_associate() ends up inserting a mapping for 
the IRQ into a radix tree rather than the linear_revmap array, but it's 
clearly wrong that the driver creates a domain of size 4 & then uses hwirq=4, 
hence the warnings.

> > > However, I *would* like to see this issue cleaned up consistently
> > > across all our drivers.  I mooted a couple ideas in [1], but nobody
> > > seemed interested.  If I merged your series as-is, there would be even
> > > less interest.
> > 
> > I've been travelling & haven't had time to look at any reworks as of yet,
> > but I do think the driver as-is is clearly broken & my fix is a pretty
> > obvious one, even if you would like the driver(s) to improve further in
> > future.
> 
> My problem is that if all the drivers work because they use 5 numbers
> (0-4), the issue will completely drop off everybody's radar.

I understand, and it's your call, but I'd argue that the driver as-is isn't 
just suboptimal but plain broken - and I think that fixing it so that it's 
"just" suboptimal is a worthwhile improvement that shouldn't be held up. But 
you're the maintainer, and if you'd like to use this to bribe me or someone 
else into improving things at some later date then so be it.

Thanks,
    Paul
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