[PATCH v2 1/3] 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.

Since we overwrite the shared host controller's can_dma callback, save
the original callback pointer during probe and restore it in the unbind
(remove) path to prevent an execute-after-free vulnerability when the
applespi driver is unloaded.

Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/input/keyboard/applespi.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/drivers/input/keyboard/applespi.c b/drivers/input/keyboard/applespi.c
index b5ff71cd5a70..07a910cb8459 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>
@@ -431,6 +432,10 @@ struct applespi_data {
 	int				tp_dim_max_x;
 	int				tp_dim_min_y;
 	int				tp_dim_max_y;
+
+	bool				(*original_can_dma)(struct spi_controller *controller,
+							    struct spi_device *spi,
+							    struct spi_transfer *xfer);
 };
 
 static const unsigned char applespi_scancodes[] = {
@@ -1605,6 +1610,13 @@ 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;
@@ -1788,6 +1800,19 @@ static int applespi_probe(struct spi_device *spi)
 	debugfs_create_file("tp_dim", 0400, applespi->debugfs_root, applespi,
 			    &applespi_tp_dim_fops);
 
+	/*
+	 * MacBook8,1's SPI host controller DMA is broken (timeout errors).
+	 * Force PIO mode by overriding the controller's can_dma callback.
+	 *
+	 * Since we modify the shared controller's callback, we save the
+	 * original pointer and restore it in applespi_remove().
+	 */
+	applespi->original_can_dma = spi->controller->can_dma;
+	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;
+	}
+
 	return 0;
 }
 
@@ -1822,6 +1847,9 @@ static void applespi_remove(struct spi_device *spi)
 
 	applespi_drain_reads(applespi);
 
+	if (applespi->original_can_dma)
+		spi->controller->can_dma = applespi->original_can_dma;
+
 	debugfs_remove_recursive(applespi->debugfs_root);
 }
 
-- 
2.39.5