[PATCH 1/1] Input: applespi - force PIO mode on MacBook8,1

[Shih-Yuan Lee <fourdollars@debian.org>]

On MacBook8,1 (early 2015 12" MacBook), the LPSS SPI controller's DMA
handshake and interrupt routing frequently fail or time out after warm
reboots, causing the keyboard and trackpad to become unresponsive and
spamming "SPI transfer timed out" (-110) errors to dmesg.

Address this by introducing a DMI quirk inside the probe function that
detects the MacBook8,1 model and overrides the controller's can_dma
callback to a custom helper that always returns false. This forces the
host controller to fall back to the rock-solid PIO mode.

This is a self-contained device quirk that completely solves the DMA
race condition on MacBook8,1 systems without impacting other MacBooks or
generic Intel platforms.

Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/input/keyboard/applespi.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/input/keyboard/applespi.c b/drivers/input/keyboard/applespi.c
index b5ff71cd5a70..edf81e383612 100644
--- a/drivers/input/keyboard/applespi.c
+++ b/drivers/input/keyboard/applespi.c
@@ -45,6 +45,7 @@
 #include <linux/crc16.h>
 #include <linux/debugfs.h>
 #include <linux/delay.h>
+#include <linux/dmi.h>
 #include <linux/efi.h>
 #include <linux/input.h>
 #include <linux/input/mt.h>
@@ -1605,6 +1606,13 @@ static void applespi_save_bl_level(struct applespi_data *applespi,
 			 "Error saving backlight level to EFI vars: 0x%lx\n", sts);
 }
 
+static bool applespi_can_not_dma(struct spi_controller *controller,
+				 struct spi_device *spi,
+				 struct spi_transfer *xfer)
+{
+	return false;
+}
+
 static int applespi_probe(struct spi_device *spi)
 {
 	struct applespi_data *applespi;
@@ -1613,6 +1621,15 @@ static int applespi_probe(struct spi_device *spi)
 	int sts, i;
 	unsigned long long gpe, usb_status;
 
+	/*
+	 * MacBook8,1's SPI host controller DMA is broken (timeout errors).
+	 * Force PIO mode by overriding the controller's can_dma callback.
+	 */
+	if (dmi_match(DMI_PRODUCT_NAME, "MacBook8,1")) {
+		dev_info(&spi->dev, "Disabling DMA for MacBook8,1 SPI to force PIO mode\n");
+		spi->controller->can_dma = applespi_can_not_dma;
+	}
+
 	/* check if the USB interface is present and enabled already */
 	acpi_sts = acpi_evaluate_integer(spi_handle, "UIST", NULL, &usb_status);
 	if (ACPI_SUCCESS(acpi_sts) && usb_status) {
-- 
2.39.5