[PATCH 4/5] drm/i915/dmc: change meaning of dmc_firmware_path="" module param

[Jani Nikula]

The distinction between the dmc_firmware_path module param being NULL
and the empty string "" is problematic. It's not possible to set the
parameter back to NULL via sysfs or debugfs. Remove the distinction, and
consider NULL and the empty string to be the same thing, and use the
platform default for them.

This removes the possibility to disable DMC (and runtime PM) via
i915.dmc_firmware_path="". Instead, you will need to specify a
non-existent file or a file that will not parse correctly.

Signed-off-by: Jani Nikula <jani.nikula at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dmc.c | 20 ++++++++++----------
 drivers/gpu/drm/i915/i915_params.c       |  3 ++-
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dmc.c b/drivers/gpu/drm/i915/display/intel_dmc.c
index 740c05ce83cc..3e510c2be1eb 100644
--- a/drivers/gpu/drm/i915/display/intel_dmc.c
+++ b/drivers/gpu/drm/i915/display/intel_dmc.c
@@ -73,6 +73,13 @@ static struct intel_dmc *i915_to_dmc(struct drm_i915_private *i915)
 	return i915->display.dmc.dmc;
 }
 
+static const char *dmc_firmware_param(struct drm_i915_private *i915)
+{
+	const char *p = i915->params.dmc_firmware_path;
+
+	return p && *p ? p : NULL;
+}
+
 #define DMC_VERSION(major, minor)	((major) << 16 | (minor))
 #define DMC_VERSION_MAJOR(version)	((version) >> 16)
 #define DMC_VERSION_MINOR(version)	((version) & 0xffff)
@@ -989,7 +996,7 @@ static void dmc_load_work_fn(struct work_struct *work)
 
 	err = request_firmware(&fw, dmc->fw_path, i915->drm.dev);
 
-	if (err == -ENOENT && !i915->params.dmc_firmware_path) {
+	if (err == -ENOENT && !dmc_firmware_param(i915)) {
 		fallback_path = dmc_fallback_path(i915);
 		if (fallback_path) {
 			drm_dbg_kms(&i915->drm, "%s not found, falling back to %s\n",
@@ -1062,15 +1069,8 @@ void intel_dmc_init(struct drm_i915_private *i915)
 
 	dmc->fw_path = dmc_firmware_default(i915, &dmc->max_fw_size);
 
-	if (i915->params.dmc_firmware_path) {
-		if (strlen(i915->params.dmc_firmware_path) == 0) {
-			drm_info(&i915->drm,
-				 "Disabling DMC firmware and runtime PM\n");
-			goto out;
-		}
-
-		dmc->fw_path = i915->params.dmc_firmware_path;
-	}
+	if (dmc_firmware_param(i915))
+		dmc->fw_path = dmc_firmware_param(i915);
 
 	if (!dmc->fw_path) {
 		drm_dbg_kms(&i915->drm,
diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
index de43048543e8..9e7f2a9f6287 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -109,7 +109,8 @@ i915_param_named_unsafe(huc_firmware_path, charp, 0400,
 	"HuC firmware path to use instead of the default one");
 
 i915_param_named_unsafe(dmc_firmware_path, charp, 0400,
-	"DMC firmware path to use instead of the default one");
+	"DMC firmware path to use instead of the default one. "
+	"Use non-existent file to disable DMC and runtime PM.");
 
 i915_param_named_unsafe(gsc_firmware_path, charp, 0400,
 	"GSC firmware path to use instead of the default one");
-- 
2.39.2