[PATCH v4 1/4] apple-gmux: use first bit to check switch state

[Orlando Chamberlain]

On T2 Macs with MMIO gmux, when GMUX_PORT_SWITCH_DISPLAY is read, it can
have values of 2, 3, 4, and 5. Odd values correspond to the discrete gpu,
and even values correspond to the integrated gpu. The current logic is
that only 2 corresponds to IGD, but this doesn't work for T2 Macs.
Instead, check the first bit to determine the connected gpu.

As T2 Macs with gmux only can switch the internal display, it is
untested if this change (or a similar change) would be applicable
to GMUX_PORT_SWITCH_DDC and GMUX_PORT_SWITCH_EXTERNAL.

Reviewed-by: Hans de Goede <hdegoede@xxxxxxxxxx>
Signed-off-by: Orlando Chamberlain <orlandoch.dev@xxxxxxxxx>
---
v3->v4: Collect Hans' review
 drivers/platform/x86/apple-gmux.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/platform/x86/apple-gmux.c b/drivers/platform/x86/apple-gmux.c
index 9333f82cfa8a..ec99e05e532c 100644
--- a/drivers/platform/x86/apple-gmux.c
+++ b/drivers/platform/x86/apple-gmux.c
@@ -346,10 +346,10 @@ static void gmux_read_switch_state(struct apple_gmux_data *gmux_data)
 	else
 		gmux_data->switch_state_ddc = VGA_SWITCHEROO_DIS;
 
-	if (gmux_read8(gmux_data, GMUX_PORT_SWITCH_DISPLAY) == 2)
-		gmux_data->switch_state_display = VGA_SWITCHEROO_IGD;
-	else
+	if (gmux_read8(gmux_data, GMUX_PORT_SWITCH_DISPLAY) & 1)
 		gmux_data->switch_state_display = VGA_SWITCHEROO_DIS;
+	else
+		gmux_data->switch_state_display = VGA_SWITCHEROO_IGD;
 
 	if (gmux_read8(gmux_data, GMUX_PORT_SWITCH_EXTERNAL) == 2)
 		gmux_data->switch_state_external = VGA_SWITCHEROO_IGD;
-- 
2.39.2