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SubjectRe: [PATCH] x86: Don't write io_apic ID if it is not changed
On Sat, Aug 14, 2010 at 01:38:01PM -0700, Yinghai Lu wrote:
>
> For 32bit mptable path, setup_ids_from_mpc() always write io apic id
> register, even there is no change needed.
>
> So try to do that when they are different bewteen reading out and mptable
>
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>
>
> ---
> arch/x86/kernel/apic/io_apic.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
> +++ linux-2.6/arch/x86/kernel/apic/io_apic.c
> @@ -2125,7 +2125,6 @@ void __init setup_ioapic_ids_from_mpc(vo
> physids_or(phys_id_present_map, phys_id_present_map, tmp);
> }
>
> -
> /*
> * We need to adjust the IRQ routing table
> * if the ID changed.
> @@ -2137,9 +2136,12 @@ void __init setup_ioapic_ids_from_mpc(vo
> = mp_ioapics[apic_id].apicid;
>
> /*
> - * Read the right value from the MPC table and
> - * write it into the ID register.
> + * Update the ID register according to the right value from
> + * the MPC table if they are different.
> */
> + if (mp_ioapics[apic_id].apicid == reg_00.bits.ID)
> + continue;
> +
> apic_printk(APIC_VERBOSE, KERN_INFO
> "...changing IO-APIC physical APIC ID to %d ...",
> mp_ioapics[apic_id].apicid);

As far as I see, this was done by a purpose in former code. Consider the
situation when mp_ioapics[apic_id].apicid >= get_physical_broadcast().
By code flow (io_apic.c:2099) this set

if (mp_ioapics[apic_id].apicid >= get_physical_broadcast()) {
printk(KERN_ERR "BIOS bug, IO-APIC#%d ID is %d in the MPC table!...\n",
apic_id, mp_ioapics[apic_id].apicid);
printk(KERN_ERR "... fixing up to %d. (tell your hw vendor)\n",
reg_00.bits.ID);
---> mp_ioapics[apic_id].apicid = reg_00.bits.ID;
}
So with your patch we always hit "continue" without real changing of ID which
is not correct.

But perhaps I miss something?

-- Cyrill


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