linux-kernel - Re: [PATCH V2 3/5] PCI: xilinx: Modifying AXI PCIe Host Bridge driver to work on both Zynq and Microblaze

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Date:	Tue, 26 Jan 2016 13:11:20 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Michal Simek <michal.simek@...inx.com>
Cc:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>,
	bhelgaas@...gle.com, lorenzo.pieralisi@....com,
	paul.burton@...tec.com, yinghai@...nel.org, wangyijing@...wei.com,
	robh@...nel.org, russell.joyce@...k.ac.uk, sorenb@...inx.com,
	jiang.liu@...ux.intel.com, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Bharat Kumar Gogada <bharatku@...inx.com>,
	Ravi Kiran Gummaluri <rgummal@...inx.com>
Subject: Re: [PATCH V2 3/5] PCI: xilinx: Modifying AXI PCIe Host Bridge driver to work on both Zynq and Microblaze

On Tuesday 26 January 2016 10:59:12 Michal Simek wrote:
> >> diff --git a/drivers/pci/host/pcie-xilinx.c b/drivers/pci/host/pcie-xilinx.c
> >> index 3e3757f..1981948 100644
> >> --- a/drivers/pci/host/pcie-xilinx.c
> >> +++ b/drivers/pci/host/pcie-xilinx.c
> >> @@ -92,7 +92,12 @@
> >>  #define ECAM_DEV_NUM_SHIFT          12
> >>  
> >>  /* Number of MSI IRQs */
> >> -#define XILINX_NUM_MSI_IRQS         128
> >> +#define XILINX_NUM_MSI_IRQS 128
> >> +#ifdef CONFIG_ARM
> >> +#define TOT_NR_IRQS         XILINX_NUM_MSI_IRQS
> >> +#else
> >> +#define TOT_NR_IRQS         (NR_IRQS + XILINX_NUM_MSI_IRQS)
> >> +#endif
> > 
> > Something looks wrong here in the microblaze variant. What does NR_IRQS
> > have to do with it?
> 
> Arnd: What was the story regarding NR_IRQS?
> I remember some discussion about it but just forget.
> 
> Default value in include/asm-generic/irq.h is 64.
> Current value is 32 because microblaze primary interrupt controller is
> axi_intc core which has up to 32 input lines.

The value in asm-generic is completely arbitrary, it's just something
that happens to work for a number of the simpler architectures.

Traditionally, there is a a fixed NR_IRQS which defines the maximum
number of interrupts that can be used, and each irqchip has a fixed
start offset below that number.

On modern systems, you have CONFIG_SPARSE_IRQ, which lets an irqchip
allocate its own interrupts, without an upper limit. This is more
flexible and avoids preallocating space for all irq_desc instances,
so it saves memory.

This code however doesn't do either of the two on microblaze:

+       irq = pos;
+#ifdef CONFIG_MICROBLAZE
+#define TOT_NR_IRQS            (NR_IRQS + XILINX_NUM_MSI_IRQS)
+       irq = XILINX_NUM_MSI_IRQS + pos;
+#endif
+       if (irq < TOT_NR_IRQS)
                set_bit(pos, msi_irq_in_use);

So you define XILINX_NUM_MSI_IRQS to mean the number of interrupts
that the xilinx_pcie_port can handle itself, but then pick a number
outside of this range by making the hwirq number something between
XILINX_NUM_MSI_IRQS and (2*XILINX_NUM_MSI_IRQS - 1), and in the
end compare it to (NR_IRQS + XILINX_NUM_MSI_IRQS), which is the
sum of two things that are not related: the total number of interrupts
including the MSIs and the number of MSI.

	Arnd