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Date:	Wed, 5 Dec 2007 05:25:54 -0800
From:	"Natalie Protasevich" <protasnb@...il.com>
To:	"Len Brown" <lenb@...nel.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Andi Kleen" <ak@...e.de>, "Ingo Molnar" <mingo@...hat.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386 IOAPIC: de-fang IRQ compression

On Nov 27, 2007 10:21 PM, Len Brown <lenb@...nel.org> wrote:
>     commit c434b7a6aedfe428ad17cd61b21b125a7b7a29ce
>     (x86: avoid wasting IRQs for PCI devices)
>     created a concept of "IRQ compression" on i386
>     to conserve IRQ numbers on systems with many
>     sparsely populated IO APICs.
>
>     The same scheme was also added to x86_64,
>     but later removed when x86_64 recieved an IRQ over-haul
>     that made it unnecessary -- including per-CPU
>     IRQ vectors that greatly increased the IRQ capacity
>     on the machine.
>
>     i386 has not received the analogous over-haul,
>     and thus a previous attempt to delete IRQ compression
>     from i386 was rejected on the theory that there may
>     exist machines that actually need it.  The fact is
>     that the author of IRQ compression patch was unable
>     to confirm the actual existence of such a system.

Those systems did exist (and still exist actually). They used over 200
irqs sometimes and with "normal" IRQ allocation they were failing even
before reaching half of their I/O configuration. So simple removal
wouldn't work for those, dynamic allocation sure would. They "scrolled
off the topic" though because new generations of such machines are not
32 bit anymore. So the author didn't actually object :) it was the
other users of large 32 bit platforms that did.

However, this patch is great, it should've been in there from the
start. However, as of today, I am not aware of systems that would
still be interested in large 32 bit configuration so wouldn't object
totall removal.

(As for irq0 workarounds - nothing special about those. They were
dictated by the code flow and the practical cases, and the fact that
is was "quick" workaround over pretty inflexible piece of code at the
time. They were truly safe no-ops for all "normal" boxes...)

>
>     As a result, all i386 kernels with IOAPIC support
>     pay the following:
>
>     1. confusion
>
>     IRQ compression re-names the traditional IOAPIC
>     pin numbers (aka ACPI GSI's) into sequential IRQ #s:
>
>     ACPI: PCI Interrupt 0000:00:1c.0[A] -> GSI 20 (level, low) -> IRQ 16
>     ACPI: PCI Interrupt 0000:00:1c.1[B] -> GSI 21 (level, low) -> IRQ 17
>     ACPI: PCI Interrupt 0000:00:1c.2[C] -> GSI 22 (level, low) -> IRQ 18
>     ACPI: PCI Interrupt 0000:00:1c.3[D] -> GSI 23 (level, low) -> IRQ 19
>     ACPI: PCI Interrupt 0000:00:1c.4[A] -> GSI 20 (level, low) -> IRQ 16
>
>     This makes /proc/interrupts look different
>     depending on system configuration and device probe order.
>     It is also different than the x86_64 kernel running
>     on the exact same system.  As a result, programmers
>     get confused when comparing systems.
>
>     2. complexity
>
>     The IRQ code in Linux is already overly complex,
>     and IRQ compression makes it worse.  There have
>     already been two bug workarounds related to IRQ
>     compression -- the IRQ0 timer workaround and
>     the VIA PCI IRQ workaround.
>
>     3. size
>
>     All i386 kernels with IOAPIC support contain an int[4096] --
>     a 4 page array to contain the renamed IRQs.
>
>     So while the irq compression code on i386 should really
>     be deleted -- even before merging the x86_64 irq-overhaul,
>     this patch simply disables it on all high volume systems
>     to avoid problems #1 and #2 on most all i386 systems.
>
>     A large system with pin numbers >=64 will still have compression
>     to conserve limited IRQ numbers for sparse IOAPICS.  However,
>     the vast majority of the planet, those with only pin numbers < 64
>     will use an identity GSI -> IRQ mapping.
>
>     Signed-off-by: Len Brown <len.brown@...el.com>
>
> diff --git a/arch/x86/kernel/mpparse_32.c b/arch/x86/kernel/mpparse_32.c
> index 7a05a7f..468d6ed 100644
> --- a/arch/x86/kernel/mpparse_32.c
> +++ b/arch/x86/kernel/mpparse_32.c
> @@ -1041,13 +1041,14 @@ void __init mp_config_acpi_legacy_irqs (void)
>  }
>
>  #define MAX_GSI_NUM    4096
> +#define IRQ_COMPRESSION_START  64
>
>  int mp_register_gsi(u32 gsi, int triggering, int polarity)
>  {
>         int ioapic = -1;
>         int ioapic_pin = 0;
>         int idx, bit = 0;
> -       static int pci_irq = 16;
> +       static int pci_irq = IRQ_COMPRESSION_START;
>         /*
>          * Mapping between Global System Interrups, which
>          * represent all possible interrupts, and IRQs
> @@ -1086,12 +1087,16 @@ int mp_register_gsi(u32 gsi, int triggering, int polarity)
>         if ((1<<bit) & mp_ioapic_routing[ioapic].pin_programmed[idx]) {
>                 Dprintk(KERN_DEBUG "Pin %d-%d already programmed\n",
>                         mp_ioapic_routing[ioapic].apic_id, ioapic_pin);
> -               return gsi_to_irq[gsi];
> +               return (gsi < IRQ_COMPRESSION_START ? gsi : gsi_to_irq[gsi]);
>         }
>
>         mp_ioapic_routing[ioapic].pin_programmed[idx] |= (1<<bit);
>
> -       if (triggering == ACPI_LEVEL_SENSITIVE) {
> +       /*
> +        * For GSI >= 64, use IRQ compression
> +        */
> +       if ((gsi >= IRQ_COMPRESSION_START)
> +               && (triggering == ACPI_LEVEL_SENSITIVE)) {
>                 /*
>                  * For PCI devices assign IRQs in order, avoiding gaps
>                  * due to unused I/O APIC pins.
>
--
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