[U-Boot] CPU Clock frequency at boot time - Nvidia Jetson TK1

[Stephen Warren]

On 08/22/2016 08:37 AM, vinoth eswaran wrote:
> Hello Developers,
>
>   Currently I am working on an embedded project using Tegra Jetson Tk1
> board. The Linux Kernel version is 4.7 and the u-boot version is v2016.05.
> I am building a customised Linux kernel and u-boot to have the fastest boot
> time possible.
>
> During Kernel startup I am seeing some error messages related to CPU clock
> frequencies like,
>
> [    0.000000] Tegra clk 127: register failed with -17
>
> [    4.275805] cpufreq: cpufreq_online: CPU0: Running at unlisted freq:
> 696000 KHz
> [    4.275845] cpufreq: cpufreq_online: CPU0: Unlisted initial frequency
> changed to: 714000 KHz
>
> On seeing the bootchart I have a feeling that the cpu clock frequency might
> be low - as lot of drivers are taking more time than usual, so I now wish
> to know what frequency does the cpu cores are running on during boot time.
> I am not clear on who is responsible(u-boot/kernel) for setting up the cpu
> clock frequency during the boot time.
>
> In u-boot which modules do I need to look to get information about cpu
> frequency being set.

This all happens on SoC-specific code; take a look at arch/arm/mach-tegra/.

I don't immediately see anything that sets a particular clock rate for 
the CPU, so I'm not sure what clock rate the CPU runs at for Tega124. 
I'm CC'ing Tom Warren in case he can recall. If you "#define DEBUG" in 
arch/arm/mach-tegra/clock.c, then clock_init() will print out the PLL 
clock rates. However, I can't guarantee it'll print the correct rates; 
it's debug code that was added early on (i.e. prior to Tegra124 support 
being added) so it's possible it hasn't been updated to get the correct 
answer on more recent SoCs. Oh, and IIRC the CPU can run on either PLL_X 
or some other PLL. You'd need to check which clock source is in use by 
reading the CAR registers; see the TRM for details of the CAR.

Also note that the memory controller clock likely boots at a non-maximal 
rate. This is controlled by the BCT.

Typically both the CPU clock and memory controller clock run at 
non-maximal rates at boot time, and hence during U-Boot. That's because 
using higher rates may (a) require PMIC programming, and (b) not be safe 
without active thermal management. I don't remember which 
frequency/voltage combinations are legal/tested/thermally-safe. If you 
need more details here, the NVIDIA L4T web forums are the best place to 
get answers; visit http://developer.nvidia.com/embedded-computing the 
follow the community link, then follow the forum link.

Once the kernel boots, it is responsible for any clock management. If 
higher-than-default clocks are required, the kernel is responsible for 
selecting these.