[PATCH] realtek: mark clock source as continuous

[Jan Hoffmann]

On 31.10.22 at 10:11, Sander Vanheule wrote:
> After replacing the R4K event timer and clock source with the new
> Realtek Otto timer, performance for RTL839x devices was severely
> impacted, as reported by Hiroshi.
> 
> Research by Markus showed that after commit 4657a5301eb5 (realtek: avoid
> busy waiting for RTL839x PHY read/write, 2022-10-14), the ethernet
> driver could only update a phy once per timer interval, which also
> heavily impacted boot time. On e.g. a Zyxel GS1900-48, this added around
> a minute to the time to fully initialise the switch.
> 
> By marking the otto clocksource as continuous, the kernel enables it to
> be used for high resolution timers. This allows readx_poll_timeout() to
> sleep for less than one system timer interval, reducing system dead
> time.

I can confirm this brings usleep_range sleep duration back to a more 
reasonable amount of time.

Average time for usleep_range(10, 20); when called in a loop 10000 times 
(tested on HPE 1920-8G for RTL838x and 1920-48G for RTL839x):

                           RTL838x     RTL839x
r4k timer                   48 us       31 us
otto timer               10003 us    10031 us
otto timer with patch       60 us       32 us

Average runtime of the polling loop in smi_wait_op is now also back to 
where it was with the r4k timer (under 0.3 ms for RTL838x and under 0.5 
ms for RTL839x on otherwise idle device, compared to 5 ms / 10 ms when 
using the otto timer without the patch).

> Link: https://github.com/openwrt/openwrt/issues/11117
> Reported-by: INAGAKI Hiroshi <musashino.open at gmail.com>
> Cc: Markus Stockhausen <markus.stockhausen at gmx.de>
> Signed-off-by: Sander Vanheule <sander at svanheule.net>
> ---
> With this patch, initialisation time for my GS1900-48 drops from 110
> seconds to 50 seconds. Please check if you can reproduce this. The 'why
> this works' from the commit message is from a quick look at the places
> where this CLOCK_SOURCE flag is checked, so I hope it actually makes sense.
> 
> Best,
> Sander
> 
>   .../realtek/files-5.10/drivers/clocksource/timer-rtl-otto.c      | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/target/linux/realtek/files-5.10/drivers/clocksource/timer-rtl-otto.c b/target/linux/realtek/files-5.10/drivers/clocksource/timer-rtl-otto.c
> index 12eed78653d0..14e28e50f40e 100644
> --- a/target/linux/realtek/files-5.10/drivers/clocksource/timer-rtl-otto.c
> +++ b/target/linux/realtek/files-5.10/drivers/clocksource/timer-rtl-otto.c
> @@ -227,6 +227,7 @@ struct rttm_cs rttm_cs = {
>   		.name	= "realtek_otto_timer",
>   		.rating	= 400,
>   		.mask	= CLOCKSOURCE_MASK(RTTM_BIT_COUNT),
> +		.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
>   		.read	= rttm_read_clocksource,
>   		.enable	= rttm_enable_clocksource
>   	}