EMI slow clock and AXI clock on i.MX6SL are changing dynamically

[jayakumar2]

Hi,

I'm bringing up a i.MX6sl board that gets interfaced using the EIM bus to a FPGA. The FPGA application runs purely in userspace. This application works but we're finding that the EIM bus cycle timing is changing dynamically resulting in sporadic failures.

After debugging, I found the following:

root@freescale ~$ mount -t debugfs debugfs /sys/kernel/debug

root@freescale ~$  find /sys/kernel/debug/clock -name \*emi\*

/sys/kernel/debug/clock/osc_clk/pll2_528_bus_main_clk/pll2_pfd2_400M/periph_clk/axi_clk/emi_slow_clk

_pfd2_400M/periph_clk/axi_clk/emi_slow_clk/rate c_clk/pll2_528_bus_main_clk/pll2_

8000000

_pfd2_400M/periph_clk/axi_clk/emi_slow_clk/rate c_clk/pll2_528_bus_main_clk/pll2_

8000000

_pfd2_400M/periph_clk/axi_clk/emi_slow_clk/rate c_clk/pll2_528_bus_main_clk/pll2_

66000000

_pfd2_400M/periph_clk/axi_clk/emi_slow_clk/rate c_clk/pll2_528_bus_main_clk/pll2_

66000000

_pfd2_400M/periph_clk/axi_clk/emi_slow_clk/rate c_clk/pll2_528_bus_main_clk/pll2_

8000000

_pfd2_400M/periph_clk/axi_clk/rate              c_clk/pll2_528_bus_main_clk/pll2_

198000000

_pfd2_400M/periph_clk/axi_clk/rate ebug/clock/osc_clk/pll2_528_bus_main_clk/pll2_

24000000

Above shows that the EIM clock (which is called EMI slow clock) is switching between 66MHz and 80MHz. This appears to be because the parent clock, AXI clock is itself switching between 198MHz and 24MHz.

Going up further shows the parent of AXI clock, periph clock is also changing:

24000000

_pfd2_400M/periph_clk/rate kernel/debug/clock/osc_clk/pll2_528_bus_main_clk/pll2_

24000000

root@freescale ~$

root@freescale ~$

_pfd2_400M/periph_clk/rate kernel/debug/clock/osc_clk/pll2_528_bus_main_clk/pll2_

396000000

_pfd2_400M/periph_clk/rate kernel/debug/clock/osc_clk/pll2_528_bus_main_clk/pll2_

396000000

_pfd2_400M/periph_clk/rate kernel/debug/clock/osc_clk/pll2_528_bus_main_clk/pll2_

24000000

I am doing the following in the board file:

        clk = clk_get(NULL, "emi_slow_clk");

        if (IS_ERR(clk))

                printk(KERN_ERR "emi_slow_clk not found\n");

        printk(KERN_ERR "checking EMI slow clock rate to be 66MHz\n");

        rate = clk_get_rate(clk);

        clk_enable(clk);

        if (rate != (66*1000*1000)) {

                printk(KERN_ERR "Warning: trying to set emi_slow_clk to 66MHz again\n");

                ret = clk_set_rate(clk, clk_round_rate(clk, 66000000));

                printk(KERN_ERR "ret=%d, newrate=%d\n", clk_get_rate(clk));

        }

But it does not look like this is sufficient to prevent the system from changing AXI clock underneath this.

I have already disabled CPUFREQ. I do not appear to be able to disable DVFS from the kernel config.

I've disabled CPUFREQ in the kernel. I couldn't find a way to easily

disable DVFS.

lucid@ubuntu:~/6sl/ltib/rpm/BUILD/linux-3.0.35$ egrep FREQ .config

CONFIG_ARCH_HAS_CPUFREQ=y

# CONFIG_CPU_FREQ is not set

lucid@ubuntu:~/6sl/ltib/rpm/BUILD/linux-3.0.35$ egrep DVFS .config

CONFIG_IMX_HAVE_PLATFORM_IMX_DVFS=y

lucid@ubuntu:~/6sl/ltib/rpm/BUILD/linux-3.0.35$ egrep RUNTIME .config

# CONFIG_PM_RUNTIME is not set

I suspect arch/arm/mach-mx6/bus_freq.c

reduce_bus_freq or one of the other functions is what is manipulating this. But I don't see a way to disable this. I'd be grateful for any suggestions/advice.

Thanks

jayakumar

[jayakumar2 · Accepted solution]

Ah, found an ugly solution:

Just force bus_freq to fail.

bus_freq_scaling_is_active = 0;

Linux clock api needs improvement to handle this kind of stuff cleanly.