drm/i915: refactor stepping info retrieval

Have only one if ladder for platforms and only one range check for
size. Makes it easier to handle new platforms. Remove the use of
negative return values in char, which might underflow to be positive for
some negative error codes.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445344713-1407-3-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_csr.c

@@ -177,28 +177,25 @@ static const struct stepping_info bxt_stepping_info[] = {
 	{'B', '0'}, {'B', '1'}, {'B', '2'}
 };
 
-static char intel_get_stepping(struct drm_device *dev)
+static const struct stepping_info *intel_get_stepping_info(struct drm_device *dev)
 {
-	if (IS_SKYLAKE(dev) && (dev->pdev->revision <
-			ARRAY_SIZE(skl_stepping_info)))
-		return skl_stepping_info[dev->pdev->revision].stepping;
-	else if (IS_BROXTON(dev) && (dev->pdev->revision <
-				ARRAY_SIZE(bxt_stepping_info)))
-		return bxt_stepping_info[dev->pdev->revision].stepping;
-	else
-		return -ENODATA;
-}
+	const struct stepping_info *si;
+	unsigned int size;
+
+	if (IS_SKYLAKE(dev)) {
+		size = ARRAY_SIZE(skl_stepping_info);
+		si = skl_stepping_info;
+	} else if (IS_BROXTON(dev)) {
+		size = ARRAY_SIZE(bxt_stepping_info);
+		si = bxt_stepping_info;
+	} else {
+		return NULL;
+	}
 
-static char intel_get_substepping(struct drm_device *dev)
-{
-	if (IS_SKYLAKE(dev) && (dev->pdev->revision <
-			ARRAY_SIZE(skl_stepping_info)))
-		return skl_stepping_info[dev->pdev->revision].substepping;
-	else if (IS_BROXTON(dev) && (dev->pdev->revision <
-			ARRAY_SIZE(bxt_stepping_info)))
-		return bxt_stepping_info[dev->pdev->revision].substepping;
-	else
-		return -ENODATA;
+	if (INTEL_REVID(dev) < size)
+		return si + INTEL_REVID(dev);
+
+	return NULL;
 }
 
 /**
@@ -285,8 +282,8 @@ static void finish_csr_load(const struct firmware *fw, void *context)
 	struct intel_package_header *package_header;
 	struct intel_dmc_header *dmc_header;
 	struct intel_csr *csr = &dev_priv->csr;
-	char stepping = intel_get_stepping(dev);
-	char substepping = intel_get_substepping(dev);
+	const struct stepping_info *stepping_info = intel_get_stepping_info(dev);
+	char stepping, substepping;
 	uint32_t dmc_offset = CSR_DEFAULT_FW_OFFSET, readcount = 0, nbytes;
 	uint32_t i;
 	uint32_t *dmc_payload;
@@ -295,11 +292,14 @@ static void finish_csr_load(const struct firmware *fw, void *context)
 	if (!fw)
 		goto out;
 
-	if ((stepping == -ENODATA) || (substepping == -ENODATA)) {
+	if (!stepping_info) {
 		DRM_ERROR("Unknown stepping info, firmware loading failed\n");
 		goto out;
 	}
 
+	stepping = stepping_info->stepping;
+	substepping = stepping_info->substepping;
+
 	/* Extract CSS Header information*/
 	css_header = (struct intel_css_header *)fw->data;
 	if (sizeof(struct intel_css_header) !=