drm/tilcdc: fix kernel panic on suspend when no hdmi monitor connected

On BeagleBone Black if no HDMI monitor is connected and suspend
is requested a kernel panic will result:

root@am335x-evm:~# echo mem > /sys/power/state
[ 65.548710] PM: Syncing filesystems ... done.
[ 65.631311] Freezing user space processes ... (elapsed 0.006 seconds) done.
[ 65.648619] Freezing remaining freezable tasks ... (elapsed 0.005 seconds) done.
[ 65.833500] Unhandled fault: external abort on non-linefetch (0x1028) at 0xfa30e004
[ 65.841692] Internal error: : 1028 [#1] SMP ARM
  <snip>
[ 66.105287] [<c03765f0>] (platform_pm_suspend) from [<c037b6d4>] (dpm_run_callback+0x34/0x70)
[ 66.114370] [<c037b6d4>] (dpm_run_callback) from [<c037ba84>] (__device_suspend+0x10c/0x2f4)
[ 66.123357] [<c037ba84>] (__device_suspend) from [<c037d004>] (dpm_suspend+0x58/0x218)
[ 66.131796] [<c037d004>] (dpm_suspend) from [<c008d948>] (suspend_devices_and_enter+0x9c/0x3c0)
[ 66.141055] [<c008d948>] (suspend_devices_and_enter) from [<c008de7c>] (pm_suspend+0x210/0x24c)
[ 66.150312] [<c008de7c>] (pm_suspend) from [<c008cabc>] (state_store+0x68/0xb8)
[ 66.158103] [<c008cabc>] (state_store) from [<c02e9654>] (kobj_attr_store+0x14/0x20)
[ 66.166355] [<c02e9654>] (kobj_attr_store) from [<c0185c70>] (sysfs_kf_write+0x4c/0x50)
[ 66.174883] [<c0185c70>] (sysfs_kf_write) from [<c018926c>] (kernfs_fop_write+0xb4/0x150)
[ 66.183598] [<c018926c>] (kernfs_fop_write) from [<c0122638>] (vfs_write+0xa8/0x180)
[ 66.191846] [<c0122638>] (vfs_write) from [<c01229f8>] (SyS_write+0x40/0x8c)
[ 66.199365] [<c01229f8>] (SyS_write) from [<c000e580>] (ret_fast_syscall+0x0/0x48)
[ 66.207426] Code: e595c210 e5932000 e59cc000 e08c2002 (e592c000)

This is because the lcdc module is not enabled when no monitor is detected
to save power.  However the suspend handler just blindly tries to save the
lcdc state by copying out the pertinent registers. However module is off
so no good things happen when you try and access it.

This patch only saves off the registers if the module is enabled, and
then only restores the registers on resume if they were saved off during
suspend.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/tilcdc/tilcdc_drv.c

@@ -582,13 +582,20 @@ static int tilcdc_pm_suspend(struct device *dev)
 
 	drm_kms_helper_poll_disable(ddev);
 
+	/* Select sleep pin state */
+	pinctrl_pm_select_sleep_state(dev);
+
+	if (pm_runtime_suspended(dev)) {
+		priv->ctx_valid = false;
+		return 0;
+	}
+
 	/* Save register state: */
 	for (i = 0; i < ARRAY_SIZE(registers); i++)
 		if (registers[i].save && (priv->rev >= registers[i].rev))
 			priv->saved_register[n++] = tilcdc_read(ddev, registers[i].reg);
 
-	/* Select sleep pin state */
-	pinctrl_pm_select_sleep_state(dev);
+	priv->ctx_valid = true;
 
 	return 0;
 }
@@ -602,10 +609,14 @@ static int tilcdc_pm_resume(struct device *dev)
 	/* Select default pin state */
 	pinctrl_pm_select_default_state(dev);
 
-	/* Restore register state: */
-	for (i = 0; i < ARRAY_SIZE(registers); i++)
-		if (registers[i].save && (priv->rev >= registers[i].rev))
-			tilcdc_write(ddev, registers[i].reg, priv->saved_register[n++]);
+	if (priv->ctx_valid == true) {
+		/* Restore register state: */
+		for (i = 0; i < ARRAY_SIZE(registers); i++)
+			if (registers[i].save &&
+			    (priv->rev >= registers[i].rev))
+				tilcdc_write(ddev, registers[i].reg,
+					     priv->saved_register[n++]);
+	}
 
 	drm_kms_helper_poll_enable(ddev);
 

drivers/gpu/drm/tilcdc/tilcdc_drv.h

@@ -67,6 +67,7 @@ struct tilcdc_drm_private {
 
 	/* register contents saved across suspend/resume: */
 	u32 saved_register[12];
+	bool ctx_valid;
 
 #ifdef CONFIG_CPU_FREQ
 	struct notifier_block freq_transition;