On Wed, 31 Oct 2007 16:39:00 +0000, Ralf Baechle <ralf@xxxxxxxxxxxxxx> wrote:
> > > The only safe but ugly workaround is to change the return from exception
> > > code to detect if the EPC is in the range startin from the condition
> > > check in the idle loop to including the WAIT instruction and if so to
> > > patch the EPC to resume execution at the condition check or the
> > > instruction following the WAIT.
> >
> > I'm also thinking of this approach. Still wondering if it is worth to
> > implement.
>
> The tickless kernel is very interesting for the low power fraction. And
> it's especially those users who would suffer most the loss of the ability
> to use the WAIT instruction. For a system running from two AAA cells the
> tradeoff is clear ... So I think it's become a must.
Then, something like this? Selecting in build-time is not so good,
but there are some CPUs which do not need this hack at all.
Synthesizing the ret_from_irq() at runtime might satisfy everyone?
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index c8c47a2..621130c 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -51,12 +51,17 @@ static void r39xx_wait(void)
* But it is implementation-dependent wheter the pipelie restarts when
* a non-enabled interrupt is requested.
*/
+#ifdef CONFIG_ROLLBACK_CPU_WAIT
+extern void cpu_wait_rollback(void);
+#define r4k_wait cpu_wait_rollback
+#else
static void r4k_wait(void)
{
__asm__(" .set mips3 \n"
" wait \n"
" .set mips0 \n");
}
+#endif
/*
* This variant is preferable as it allows testing need_resched and going to
diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index e29598a..ffa043c 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -27,6 +27,20 @@
#endif
.text
+#ifdef CONFIG_ROLLBACK_CPU_WAIT
+ .align 6
+FEXPORT(cpu_wait_rollback)
+ LONG_L t0, TI_FLAGS($28)
+ andi t0, _TIF_NEED_RESCHED
+ bnez t0, 1f
+ .set mips3
+ wait
+ .set mips0
+1:
+ jr ra
+ .align 6
+cpu_wait_rollback_end:
+#endif
.align 5
#ifndef CONFIG_PREEMPT
FEXPORT(ret_from_exception)
@@ -35,6 +49,14 @@ FEXPORT(ret_from_exception)
#endif
FEXPORT(ret_from_irq)
LONG_S s0, TI_REGS($28)
+#ifdef CONFIG_ROLLBACK_CPU_WAIT
+ LONG_L t0, PT_EPC(sp)
+ ori t0, 0x3f
+ xori t0, 0x3f
+ PTR_LA t1, cpu_wait_rollback
+ bne t0, t1, __ret_from_irq
+ LONG_S t0, PT_EPC(sp) # return to cpu_wait_rollback
+#endif
FEXPORT(__ret_from_irq)
LONG_L t0, PT_STATUS(sp) # returning to kernel mode?
andi t0, t0, KU_USER
