Hi Quentin,
On 4/23/24 11:55, Quentin Schulz wrote:
Hi Peter,
On 4/23/24 10:10, Peter Hoyes wrote:
From: Peter Hoyes <peter.ho...@arm.com>
Polling cntpct_el0 in a tight loop for delays is inefficient.
This is particularly apparent on Arm FVPs, which do not simulate
real time, meaning that a 1s sleep can take a couple of orders
of magnitude longer to execute in wall time.
If running at EL2 or above (where CNTHCTL_EL2 is available), enable
the cntpct_el0 event stream temporarily and use wfe to implement
the delay more efficiently. The event period is chosen as a
trade-off between efficiency and the fact that Arm FVPs do not
typically simulate real time.
This is only implemented for Armv8 boards, where an architectural
timer exists.
Some mach-socfpga AArch64 boards already override __udelay to make
it always inline, so guard the functionality with a new
ARMV8_UDELAY_EVENT_STREAM Kconfig, enabled by default.
Signed-off-by: Peter Hoyes <peter.ho...@arm.com>
---
arch/arm/cpu/armv8/Kconfig | 8 ++++++++
arch/arm/cpu/armv8/generic_timer.c | 27 +++++++++++++++++++++++++++
arch/arm/include/asm/system.h | 6 ++++--
3 files changed, 39 insertions(+), 2 deletions(-)
diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
index 9f0fb369f7..544c5e2d74 100644
--- a/arch/arm/cpu/armv8/Kconfig
+++ b/arch/arm/cpu/armv8/Kconfig
@@ -191,6 +191,14 @@ config ARMV8_EA_EL3_FIRST
Exception handling at all exception levels for External Abort
and
SError interrupt exception are taken in EL3.
+config ARMV8_UDELAY_EVENT_STREAM
+ bool "Use the event stream for udelay"
+ default y if !ARCH_SOCFPGA
+ help
+ Use the event stream provided by the AArch64 architectural
timer for
+ delays. This is more efficient than the default polling
+ implementation.
+
menuconfig ARMV8_CRYPTO
bool "ARM64 Accelerated Cryptographic Algorithms"
diff --git a/arch/arm/cpu/armv8/generic_timer.c
b/arch/arm/cpu/armv8/generic_timer.c
index 8f83372cbc..e18b5c8187 100644
--- a/arch/arm/cpu/armv8/generic_timer.c
+++ b/arch/arm/cpu/armv8/generic_timer.c
@@ -115,3 +115,30 @@ ulong timer_get_boot_us(void)
return val / get_tbclk();
}
+
+#if CONFIG_IS_ENABLED(ARMV8_UDELAY_EVENT_STREAM)
+void __udelay(unsigned long usec)
+{
+ u64 target = get_ticks() + usec_to_tick(usec);
+
This can theoretically overflow, do we have any guarantee this cannot
happen in real life, like... we would need U-Boot to be running for
100 years without being powered-down/reset or something like that? Can
we document this assumption? Does this make sense?
+ /* At EL2 or above, use the event stream to avoid polling
CNTPCT_EL0 so often */
+ if (current_el() >= 2) {
+ u32 cnthctl_val;
+ const u8 event_period = 0x7;
+
+ asm volatile("mrs %0, cnthctl_el2" : "=r" (cnthctl_val));
+ asm volatile("msr cnthctl_el2, %0" : : "r"
+ (cnthctl_val | CNTHCTL_EL2_EVNT_EN |
CNTHCTL_EL2_EVNT_I(event_period)));
+
+ while (get_ticks() + (1ULL << event_period) <= target)
This could be an overflow as well.
+ wfe();
+
+ /* Reset the event stream */
+ asm volatile("msr cnthctl_el2, %0" : : "r" (cnthctl_val));
+ }
+
+ /* Fall back to polling CNTPCT_EL0 */
+ while (get_ticks() <= target)
get_ticks() could wrap around here maybe?
I don't see a problem here. It's using u64s and the maximum
COUNTER_FREQUENCY in the U-Boot source is 200MHz, which gives enough
ticks for about 3 millennia.
Additionally, the underlying "while" condition is the same as the
existing weak __udelay implementation
(https://source.denx.de/u-boot/u-boot/-/blob/master/lib/time.c?ref_type=heads#L181),
so this change doesn't affect the overflow limit.
Thanks,
Peter
