Re: [dpdk-dev] [RFC 1/5] eal: add the APIs to wait until equal

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

Hi Gavin,

>-----Original Message-----
>From: dev <dev-bounces@dpdk.org> On Behalf Of Gavin Hu
>Sent: Sunday, June 30, 2019 9:51 PM
>To: dev@dpdk.org
>Cc: thomas@monjalon.net; Jerin Jacob Kollanukkaran
><jerinj@marvell.com>; hemant.agrawal@nxp.com;
>bruce.richardson@intel.com; chaozhu@linux.vnet.ibm.com;
>Honnappa.Nagarahalli@arm.com; nd@arm.com; gavin.hu@arm.com
>Subject: [dpdk-dev] [RFC 1/5] eal: add the APIs to wait until equal
>
>The rte_wait_until_equal_xxx APIs abstract the functionality of 'polling
>for a memory location to become equal to a given value'.
>
>Signed-off-by: Gavin Hu <gavin.hu@arm.com>
>Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
>Reviewed-by: Steve Capper <steve.capper@arm.com>
>Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
>Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
>---
> .../common/include/arch/arm/rte_pause_64.h         | 143
>+++++++++++++++++++++
> lib/librte_eal/common/include/generic/rte_pause.h  |  20 +++
> 2 files changed, 163 insertions(+)
>
>diff --git a/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
>b/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
>index 93895d3..0095da6 100644
>--- a/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
>+++ b/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
>@@ -17,6 +17,149 @@ static inline void rte_pause(void)
> 	asm volatile("yield" ::: "memory");
> }
>
>+#ifdef RTE_USE_WFE
>+#define rte_wait_until_equal_relaxed(addr, expected) do {\
>+		typeof(*addr) tmp;  \
>+		if (__builtin_constant_p((expected))) \
>+			do { \
>+				if (sizeof(*(addr)) == 16)\
>+					asm volatile(  \
>+						"sevl\n"  \
>+						"1:	 wfe\n"  \
>+						"ldxrh  %w0, %1\n"  \
>+						"cmp	%w0, %w2\n"  \
>+						"bne	1b\n"  \
>+						: "=&r"(tmp)  \
>+						: "Q"(*addr),
>"i"(expected)  \
>+						: "cc", "memory");  \

Can we have early exit here i.e. instead of going directly to wfe can we first check the condition and then fallthrough?
Something like:
		asm volatile("	ldxrh	%w0 %1	\n"
			        "    cmp	%w0 %w2	\n"
			        "	b.eq  	2:		\n"
			        "1: wfe			\n"
			        "    ldxrh	%w0, %1	\n"  
			        "    cmp	%w0, %w2	\n"  
			        "    b.ne	1b		\n"  
			        "2:				\n"
			        :::);

Regards,
Pavan.