When executing clk_disable_unprepare() in __fclk_set_enable(), the warning “pl0_ref already disabled” occurs.

[ikwzm]

The following warning appears when loading fclkcfg.

[ 1544.942268] fclkcfg: loading out-of-tree module taints kernel.
[ 1544.949984] ------------[ cut here ]------------
[ 1544.954625] pl0_ref already disabled
[ 1544.958243] WARNING: CPU: 3 PID: 558 at drivers/clk/clk.c:1181 clk_core_disable+0xb0/0xb8
[ 1544.966435] Modules linked in: fclkcfg(O+) fuse ip_tables x_tables
[ 1544.975829] CPU: 3 PID: 558 Comm: systemd-udevd Tainted: G           O       6.6.40-zynqmp-fpga-generic #1
[ 1544.985477] Hardware name: Avnet Ultra96-V2 Rev1 (DT)
[ 1544.990513] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1545.002143] pc : clk_core_disable+0xb0/0xb8
[ 1545.006317] lr : clk_core_disable+0xb0/0xb8
[ 1545.010493] sp : ffffffc081f1b600
[ 1545.013791] x29: ffffffc081f1b600 x28: 0000000000000000 x27: ffffffc081f1bcd8
[ 1545.020917] x26: ffffffc081f1bcd8 x25: ffffffc081f1b6f8 x24: 0000000000000001
[ 1545.028043] x23: ffffffc081f1b6f8 x22: ffffff8003f53de0 x21: ffffff8004906ba8
[ 1545.035170] x20: ffffff8001e45e00 x19: ffffff8001e45e00 x18: 0000000000000010
[ 1545.042296] x17: 0000000000000124 x16: ffffffc079648638 x15: ffffff80025b4a10
[ 1545.049422] x14: 00000000000001b9 x13: ffffff80025b4a10 x12: 00000000ffffffea
[ 1545.056549] x11: 00000000fffff7ff x10: 00000000fffff7ff x9 : ffffffc0814ec360
[ 1545.063675] x8 : 000000000000bfe8 x7 : c0000000fffff7ff x6 : 0000000000000001
[ 1545.070801] x5 : ffffff807fb9fac8 x4 : 0000000000000000 x3 : 0000000000000027
[ 1545.077927] x2 : ffffff807fb9fad0 x1 : 0000000000000000 x0 : 0000000000000000
[ 1545.085055] Call trace:
[ 1545.087486]  clk_core_disable+0xb0/0xb8
[ 1545.091312]  clk_core_disable_lock+0x24/0x3c
[ 1545.095574]  clk_disable+0x20/0x30
[ 1545.098968]  __fclk_change_state+0x1f8/0x2c0 [fclkcfg]
[ 1545.104115]  fclk_device_setup+0x10c/0x358 [fclkcfg]
[ 1545.109080]  fclkcfg_platform_driver_probe+0x108/0x4bc [fclkcfg]
[ 1545.115087]  platform_probe+0x68/0xd8
[ 1545.118741]  really_probe+0x110/0x280
[ 1545.122395]  __driver_probe_device+0x78/0x12c
[ 1545.126744]  driver_probe_device+0x3c/0x118
[ 1545.130919]  __driver_attach+0x74/0x124
[ 1545.134747]  bus_for_each_dev+0x78/0xdc
[ 1545.138575]  driver_attach+0x24/0x30
[ 1545.142142]  bus_add_driver+0xe4/0x208
[ 1545.145883]  driver_register+0x60/0x128
[ 1545.149711]  __platform_driver_register+0x28/0x34
[ 1545.154407]  fclkcfg_module_init+0x84/0x1000 [fclkcfg]
[ 1545.159545]  do_one_initcall+0x70/0x1b8
[ 1545.163373]  do_init_module+0x58/0x1e4
[ 1545.167115]  load_module+0x1984/0x1a4c
[ 1545.170856]  init_module_from_file+0x88/0xc8
[ 1545.175117]  __arm64_sys_finit_module+0x1ec/0x320
[ 1545.179813]  invoke_syscall+0x50/0x118
[ 1545.183555]  el0_svc_common.constprop.0+0xc0/0xe0
[ 1545.188251]  do_el0_svc+0x1c/0x28
[ 1545.191557]  el0_svc+0x40/0xe8
[ 1545.194604]  el0t_64_sync_handler+0xc0/0xc4
[ 1545.198779]  el0t_64_sync+0x190/0x194
[ 1545.202435] ---[ end trace 0000000000000000 ]---
[ 1545.207121] ------------[ cut here ]------------
[ 1545.211916] pl0_ref already unprepared
[ 1545.215805] WARNING: CPU: 3 PID: 558 at drivers/clk/clk.c:1040 clk_core_unprepare+0xf0/0x110
[ 1545.224266] Modules linked in: u_dma_buf(O+) fclkcfg(O+) fuse ip_tables x_tables
[ 1545.231671] CPU: 3 PID: 558 Comm: systemd-udevd Tainted: G        W  O       6.6.40-zynqmp-fpga-generic #1
[ 1545.241320] Hardware name: Avnet Ultra96-V2 Rev1 (DT)
[ 1545.246357] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1545.256857] pc : clk_core_unprepare+0xf0/0x110
[ 1545.261301] lr : clk_core_unprepare+0xf0/0x110
[ 1545.265737] sp : ffffffc081f1b610
[ 1545.269035] x29: ffffffc081f1b610 x28: 0000000000000000 x27: ffffffc081f1bcd8
[ 1545.276170] x26: ffffffc081f1bcd8 x25: ffffffc081f1b6f8 x24: 0000000000000001
[ 1545.283296] x23: ffffffc081f1b6f8 x22: ffffff8003f53de0 x21: ffffff8004906ba8
[ 1545.290422] x20: ffffff8004906b80 x19: ffffff8001e45e00 x18: 0000000000000010
[ 1545.297549] x17: 0000000000000124 x16: ffffffc079648638 x15: ffffff80025b4a10
[ 1545.304675] x14: 00000000000001eb x13: ffffff80025b4a10 x12: 00000000ffffffea
[ 1545.311801] x11: 00000000fffff7ff x10: 00000000fffff7ff x9 : ffffffc0814ec360
[ 1545.318928] x8 : 000000000000bfe8 x7 : c0000000fffff7ff x6 : 0000000000000001
[ 1545.326054] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
[ 1545.333180] x2 : ffffff807fba2c98 x1 : 0000000000000000 x0 : 0000000000000000
[ 1545.340307] Call trace:
[ 1545.342738]  clk_core_unprepare+0xf0/0x110
[ 1545.346825]  clk_unprepare+0x2c/0x44
[ 1545.350392]  __fclk_change_state+0x200/0x2c0 [fclkcfg]
[ 1545.355540]  fclk_device_setup+0x10c/0x358 [fclkcfg]
[ 1545.360505]  fclkcfg_platform_driver_probe+0x108/0x4bc [fclkcfg]
[ 1545.366512]  platform_probe+0x68/0xd8
[ 1545.370166]  really_probe+0x110/0x280
[ 1545.373820]  __driver_probe_device+0x78/0x12c
[ 1545.378169]  driver_probe_device+0x3c/0x118
[ 1545.382344]  __driver_attach+0x74/0x124
[ 1545.386172]  bus_for_each_dev+0x78/0xdc
[ 1545.389999]  driver_attach+0x24/0x30
[ 1545.393567]  bus_add_driver+0xe4/0x208
[ 1545.397308]  driver_register+0x60/0x128
[ 1545.401136]  __platform_driver_register+0x28/0x34
[ 1545.405832]  fclkcfg_module_init+0x84/0x1000 [fclkcfg]
[ 1545.410970]  do_one_initcall+0x70/0x1b8
[ 1545.414798]  do_init_module+0x58/0x1e4
[ 1545.418540]  load_module+0x1984/0x1a4c
[ 1545.422281]  init_module_from_file+0x88/0xc8
[ 1545.426542]  __arm64_sys_finit_module+0x1ec/0x320
[ 1545.431238]  invoke_syscall+0x50/0x118
[ 1545.434980]  el0_svc_common.constprop.0+0xc0/0xe0
[ 1545.439675]  do_el0_svc+0x1c/0x28
[ 1545.442982]  el0_svc+0x40/0xe8
[ 1545.446038]  el0t_64_sync_handler+0xc0/0xc4
[ 1545.450212]  el0t_64_sync+0x190/0x194
[ 1545.453868] ---[ end trace 0000000000000000 ]---
[ 1545.458883] fclkcfg amba_pl@0:fclk0: driver version : 1.8.0
[ 1545.464478] fclkcfg amba_pl@0:fclk0: device name    : amba_pl@0:fclk0
[ 1545.470931] fclkcfg amba_pl@0:fclk0: clock  name    : pl0_ref
[ 1545.476680] fclkcfg amba_pl@0:fclk0: clock  rate    : 249999998
[ 1545.482622] fclkcfg amba_pl@0:fclk0: clock  enabled : 1
[ 1545.487841] fclkcfg amba_pl@0:fclk0: remove rate    : 1000000
[ 1545.493587] fclkcfg amba_pl@0:fclk0: remove enable  : 0
[ 1545.498813] fclkcfg amba_pl@0:fclk0: driver installed.

[ikwzm]

fclkcfg stops the clock once when changing the clock frequency.
When stopping the clock, in fclkcfg.c, it calls clk_disable_prepare() to stop the clock if it is enabled, as follows

static int __fclk_set_enable(struct fclk_device_data* this, bool enable)
{
    int status = 0;

    if (enable == true) {
        if (__clk_is_enabled(this->clk) == false) {
            status = clk_prepare_enable(this->clk);
            if (status) 
                dev_err(this->device, "enable failed.");
            else 
                DEV_DBG(this->device, "enable success.");
        }
    } else {
        if (__clk_is_enabled(this->clk) == true) {
            clk_disable_unprepare(this->clk);
            DEV_DBG(this->device, "disable done.");
        }
    }
    return status;
}

Then, clk_disable_prepare() in linux kernel driver (drivers/clk/clk.c) checks enable_count and issues a warning if enable_count is 0 as follows

static void clk_core_disable(struct clk_core *core)
{
	lockdep_assert_held(&enable_lock);

	if (!core)
		return;

	if (WARN(core->enable_count == 0, "%s already disabled\n", core->name))
		return;

	if (WARN(core->enable_count == 1 && core->flags & CLK_IS_CRITICAL,
	    "Disabling critical %s\n", core->name))
		return;

	if (--core->enable_count > 0)
		return;

	trace_clk_disable(core);

	if (core->ops->disable)
		core->ops->disable(core->hw);

	trace_clk_disable_complete(core);

	clk_core_disable(core->parent);
}

On the other hand, zynqmp_clk_gate_is_enabled() in drivers/clk/zynqmp/clk-gate-zynqmp.c, the clock driver for ZynqMP on Linux Kernel, asks firmware as follows.

static int zynqmp_clk_gate_is_enabled(struct clk_hw *hw)
{
	struct zynqmp_clk_gate *gate = to_zynqmp_clk_gate(hw);
	const char *clk_name = clk_hw_get_name(hw);
	u32 clk_id = gate->clk_id;
	int state, ret;

	ret = zynqmp_pm_clock_getstate(clk_id, &state);
	if (ret) {
		pr_debug("%s() clock get state failed for %s, ret = %d\n",
			 __func__, clk_name, ret);
		return -EIO;
	}

	return state ? 1 : 0;
}

These things lead to a situation where __clk_is_enabled() is true but enable_count is 0 if the clock was previously enabled by firmware when ZynqMP was started up.
This is why the warning occurs.

[ikwzm]

In such a case, an inconsistency with the clock driver may occur, resulting in warning such as issue #3 when the clock is stopped.

The enable-sync property forces the clock state to be synchronized at dirver load time to suppress the occurrence of this warning.

        fclk0 {
            compatible    = "ikwzm,fclkcfg";
            device-name   = "fpga-clk0";
            clocks        = <&clkc 15>, <&clkc 2>;
            enable-sync;
            insert-enable = <0>;
        };