From 5788f77959110480486c441b912da5c2cc5fc4d2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damien=20Th=C3=A9bault?= <damien@dtbo.net>
Date: Wed, 18 Jul 2018 12:06:01 +0200
Subject: [PATCH] platform/x86: dell-laptop: Fix backlight detection
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Git-commit: 5788f77959110480486c441b912da5c2cc5fc4d2
No-fix: 2502e5a025935b7b476b69eda4959b3c4022c72b
Patch-mainline: v4.19-rc1
References: FATE#325842

Fix return code check for "max brightness" ACPI call.

The Dell laptop ACPI video brightness control is not present on dell
laptops anymore, but was present in older kernel versions.

The code that checks the return value is incorrect since the SMM
refactoring.

The old code was:
  if (buffer->output[0] == 0)

Which was changed to:
  ret = dell_send_request(...)
  if (ret)

However, dell_send_request() will return 0 if buffer->output[0] == 0,
so we must change the check to:
  if (ret == 0)

This issue was found on a Dell M4800 laptop, and the fix tested on it
as well.

Fixes: 549b4930f057 ("dell-smbios: Introduce dispatcher for SMM calls")
Signed-off-by: Damien Thébault <damien@dtbo.net>
Tested-by: Damien Thébault <damien@dtbo.net>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/platform/x86/dell-laptop.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/platform/x86/dell-laptop.c b/drivers/platform/x86/dell-laptop.c
index f1fa8612db40..06978c14c83b 100644
--- a/drivers/platform/x86/dell-laptop.c
+++ b/drivers/platform/x86/dell-laptop.c
@@ -2185,7 +2185,7 @@ static int __init dell_init(void)
 		dell_fill_request(&buffer, token->location, 0, 0, 0);
 		ret = dell_send_request(&buffer,
 					CLASS_TOKEN_READ, SELECT_TOKEN_AC);
-		if (ret)
+		if (ret == 0)
 			max_intensity = buffer.output[3];
 	}
 
-- 
2.19.1