[PATCH] serial: ns16550: Try get serial clock rate from DT before CLK

[Jonas Karlman]

Hi Quentin,

On 2024-08-05 09:19, Quentin Schulz wrote:
> Hi Jonas,
> 
> On 8/4/24 5:09 PM, Jonas Karlman wrote:
>> Initializing a clock driver to read a known static clock rate can take
>> some time at U-Boot proper pre-reloc phase.
>>
>> Change to first try and read clock rate from DT to speed up boot time,
>> fall back to getting the clock rate from clock driver.
>>
>> This help reduce boot time by around:
>> - ~35ms on a Radxa ROCK Pi 4 (RK3399)
>> - ~15ms on a Radxa ZERO 3W (RK3566)
>> Time that is wasted getting a static rate known at compile time.
>>
> 
> I guess this also makes some board perform worse as well, the ones 
> without a clock-frequency set?

I do not expect that this change will add anything to existing boot
time, however in some circumstances it may reduce the boot time.

Before this change the rate was retrieved using first successful of:

1. clk_get_rate()
2. clock-frequency prop from DT
3. CFG_SYS_NS16550_CLK

This patch swap 1. and 2. so driver first will try to use any rate that
may have been defined in DT, thus possible skip a probe of the clock.

Boot time is only saved/reduced if the serial driver was the only device
that needed something from clock device in current phase.

As long as the DT contain correct rate in clock-frequency prop or no
such prop this patch is not expected to change anything.

Regards,
Jonas

> 
> The change seems sound to me, therefore
> Reviewed-by: Quentin Schulz <quentin.schulz at cherry.de>
> 
> Thanks!
> Quentin