platform/x86: dell-laptop: Fix backlight detection

BugLink: https://bugs.launchpad.net/bugs/1835845
[ Upstream commit 2502e5a025935b7b476b69eda4959b3c4022c72b ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/platform/x86/dell-laptop.c b/drivers/platform/x86/dell-laptop.c
index 8ddb309cfc857d11b005f75d2b83d65a7f632e1b..e8d7e2f0ca10b5966cfeca04e2fb6e3e033b635c 100644 (file)
--- a/drivers/platform/x86/dell-laptop.c
+++ b/drivers/platform/x86/dell-laptop.c
@@ -2164,7 +2164,7 @@ 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];
        }