Re: [PATCH 23/24] alpha: select SPARSE_IRQ

[Magnus Lindholm]

Hi Matt,

On Tue, Sep 1, 2026 at 5:52 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> NR_IRQS on Alpha is sized for the largest configuration each platform
> supports, and for Marvel that is 32 PIDs, or 32784 interrupts.  Without
> SPARSE_IRQ that becomes a static irq_desc[] array of 12.6 MB in .data,
> carried by every Marvel and every generic kernel regardless of how many
> interrupts the machine actually has.  A real ES47 uses 87 of them.
>
> That is not just wasted memory, it constrains where the kernel can be
> loaded.  The image links at a fixed 0xfffffc0001010000, just above the
> console's low memory reservation, and with memory striping enabled the
> console leaves only about 16 MB of usable low memory before the rest is
> relocated to an interleaved aperture.  A 12.6 MB array of mostly unused
> interrupt descriptors is the difference between fitting and not.
>
> Alpha already goes through irq_to_desc() rather than indexing irq_desc[]
> directly, so nothing stands in the way.  The platforms do wire up their
> interrupts by number, though, which needs the descriptors to exist by
> then, so provide arch_probe_nr_irqs() returning the machine vector's
> count instead of taking the NR_IRQS_LEGACY default.  Without it every
> interrupt above 15 would have no descriptor and the handler setup would
> be silently dropped.
>
> Tested on an ES47 (Marvel, 2 EV7z): boots with all device interrupts
> working, and .data drops by 12.6 MB, from 15.7 MB to 3.1 MB.
> ---
>  arch/alpha/Kconfig            |  1 +
>  arch/alpha/kernel/irq_alpha.c | 13 +++++++++++++
>  2 files changed, 14 insertions(+)
>

This patch appears to be missing your Signed-off-by tag.

Tested on a two-CPU UP2000 with CONFIG_SPARSE_IRQ=y. The kernel booted
successfully and created IRQ descriptors 0 through 63. SCSI devices on
IRQs 19 and 27 and Ethernet on IRQ 43 operated normally, with their
interrupt counts increasing. I observed no IRQ setup failures or other
IRQ-related warnings.

Regards

Magnus