[PATCH 07/13] platform/apple-gmux: Add check to detect FSS

[Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>]

On MMIO macs, there is feature called FSS, we assume its
a display syncing technology between internal links
(for example 5K iMacs).

We detect it and then warn the user about it, if its broken/unavailable.

Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
---
 drivers/platform/x86/apple-gmux.c | 2 ++
 include/linux/apple-gmux.h        | 1 +
 2 files changed, 3 insertions(+)

diff --git a/drivers/platform/x86/apple-gmux.c b/drivers/platform/x86/apple-gmux.c
index 6a9580f27..dda281f2f 100644
--- a/drivers/platform/x86/apple-gmux.c
+++ b/drivers/platform/x86/apple-gmux.c
@@ -1224,6 +1224,8 @@ static int gmux_probe(struct pnp_dev *pnp, const struct pnp_device_id *id)
 	else if (boot_gpu == 1)
 		gmux_switchto(VGA_SWITCHEROO_DIS);
 
+	if (type == APPLE_GMUX_TYPE_MMIO && gmux_read8(gmux_data, GMUX_PORT_FSS) & 1)
+		pr_warn("FSS Failure, display tearing expected\n");
 	/*
 	 * Retina MacBook Pros cannot switch the panel's AUX separately
 	 * and need eDP pre-calibration. They are distinguishable from
diff --git a/include/linux/apple-gmux.h b/include/linux/apple-gmux.h
index 206d97ffd..a4fa84500 100644
--- a/include/linux/apple-gmux.h
+++ b/include/linux/apple-gmux.h
@@ -22,6 +22,7 @@
 #define GMUX_PORT_VERSION_RELEASE	0x06
 #define GMUX_PORT_SWITCH_DISPLAY	0x10
 #define GMUX_PORT_SWITCH_GET_DISPLAY	0x11
+#define GMUX_PORT_FSS			0x12
 #define GMUX_PORT_INTERRUPT_ENABLE	0x14
 #define GMUX_PORT_INTERRUPT_STATUS	0x16
 #define GMUX_PORT_SWITCH_DDC		0x28
-- 
2.55.0