Re: [PATCH RT] rtc: Disable RTC_DRV_EFI on RT

[Sebastian Andrzej Siewior]

On 2018-07-26 15:13:23 [+0200], To Ard Biesheuvel wrote:
> On 2018-07-26 14:52:21 [+0200], Ard Biesheuvel wrote:
> > We could also make it the default on -rt, but not disable it entirely, so that efi=runtime can be used to re-enable it.
> 
> Oh. I like that. We have something similar for RCU. So I would need
> that:
and then I could make it default off:

----- >8

Subject: [PATCH] efi: Disable runtime services on RT

Based on meassurements the EFI functions get_variable /
get_next_variable take up to 2us which looks okay.
The functions get_time, set_time take around 10ms. Those 10ms are too
much. Even one ms would be too much.
Ard mentioned that SetVariable might even trigger larger latencies if
the firware will erase flash blocks on NOR.

The time-functions are used by efi-rtc and can be triggered during
runtimed (either via explicit read/write or ntp sync).

The variable write could be used by pstore.
These functions can be disabled without much of a loss. The poweroff /
reboot hooks may be provided by PSCI.

Disable EFI's runtime wrappers.

This was observed on "EFI v2.60 by SoftIron Overdrive 1000".

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 drivers/firmware/efi/efi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 232f4915223b..62c6e4b6ce3e 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -84,7 +84,7 @@ struct mm_struct efi_mm = {
 	.mmlist			= LIST_HEAD_INIT(efi_mm.mmlist),
 };
 
-static bool disable_runtime;
+static bool disable_runtime = IS_ENABLED(CONFIG_PREEMPT_RT_BASE);
 static int __init setup_noefi(char *arg)
 {
 	disable_runtime = true;
-- 
2.18.0