On Tue, 12 Nov 2013, Russell King - ARM Linux wrote:
> On Tue, Nov 12, 2013 at 09:01:16AM -0500, Nicolas Pitre wrote:
> > What about this patch which I think is currently your best option. Note
> > it would need to use the facilities from asm/opcodes.h to make it endian
> > agnostic.
> >
> > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> > index 6a1b8a81b1..379cffe4ab 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -383,6 +383,34 @@ static void __init cpuid_init_hwcaps(void)
> > elf_hwcap |= HWCAP_IDIVT;
> > }
> >
> > + /*
> > + * Patch our division routines with the corresponding opcode
> > + * if the hardware supports it.
> > + */
> > + if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVT)) {
> > + extern char __aeabi_uidiv, __aeabi_idiv;
> > + u16 *uidiv = (u16 *)&__aeabi_uidiv;
> > + u16 *idiv = (u16 *)&__aeabi_idiv;
> > +
> > + uidiv[0] = 0xfbb0; /* udiv r0, r0, r1 */
> > + uidiv[1] = 0xf0f1;
> > + uidiv[2] = 0x4770; /* bx lr */
> > +
> > + idiv[0] = 0xfb90; /* sdiv r0, r0, r1 */
> > + idiv[1] = 0xf0f1;
> > + idiv[2] = 0x4770; /* bx lr */
> > + } else if (!IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap &
> > HWCAP_IDIVA)) {
> > + extern char __aeabi_uidiv, __aeabi_idiv;
> > + u32 *uidiv = (u32 *)&__aeabi_uidiv;
> > + u32 *idiv = (u32 *)&__aeabi_idiv;
> > +
> > + uidiv[0] = 0xe730f110; /* udiv r0, r0, r1 */
> > + uidiv[1] = 0xe12fff1e; /* bx lr */
> > +
> > + idiv[0] = 0xe710f110; /* sdiv r0, r0, r1 */
> > + idiv[1] = 0xe12fff1e; /* bx lr */
> > + }
>
> What about endianness, and what if XIP is enabled?
Just as I said above the diff: this needs refined.
Obviously XIP can't use this and doesn't need it either as a XIP kernel
should be optimized for the very platform it will run onto i.e. gcc
should already emit those div opcodes inline if appropriate.
Nicolas
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