Re: [RFC PATCH] arm/gic: Optimize lr_mask type based on GIC version

["Halder, Ayan Kumar" <ayankuma@xxxxxxx>]

Hi Andrew,

On 05/03/2026 23:01, Andrew Cooper wrote:
> On 05/03/2026 7:57 pm, Ayan Kumar Halder wrote:
> > The lr_mask bitmap tracks which List Registers (LRs) are in use for
> > virtual interrupt injection. Previously, lr_mask always used uint64_t
> > (8 bytes) to support the maximum number of LRs across both GIC versions.
> >
> > However, GICv2 and GICv3 have different hardware limits:
> > - GICv3: ICH_VTR_EL2[3:0] encodes LR count -> max 16 LRs (4 bits)
> > - GICv2: GICH_VTR[5:0] encodes LR count -> max 64 LRs (6 bits)
> >
> > This patch introduces conditional compilation to optimize lr_mask size:
> > - CONFIG_GICV3=y: Use uint16_t (2 bytes) - sufficient for 16 LRs
> > - CONFIG_GICV3=n: Use uint64_t (8 bytes) - required for 64 LRs
> >
> > With this, parameter 'lr' in gicv3_ich_read_lr(), gicv3_ich_write_lr()
> > cannot have a value > 15. Thus, it should not possible to hit the
> > BUG() in the default case.
> >
> > Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> What does this actually get you?
>
> Because it doesn't actually eliminate the BUG()s you reference.
As lr is obtained from lr_mask, see the snippet from gic_find_unused_lr()

        for_each_set_bit ( used_lr, *lr_mask )
        {
            struct gic_lr lr_val;

            gic_hw_ops->read_lr(used_lr, &lr_val);
            if ( lr_val.virq == p->irq )
                return used_lr;
        }

If lr_mask is 16 bits, then used_lr should not exceed 15. That is my 
thinking.
> If you really want to go ahead with this patch, then make a  lr_mask_t
> or the right type and don't double-code everything.
Ack

- Ayan