drm/sun4i: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
switches to using a kmalloc allocation and moves all the size calculations
to the start to do an allocation. If an upper bounds on the mode timing
calculations could be determined, a fixed stack size could be used instead.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180629184740.GA37415@beast

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -13,6 +13,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/regmap.h>
 #include <linux/reset.h>
+#include <linux/slab.h>
 
 #include <linux/phy/phy.h>
 
@@ -247,10 +248,8 @@ static u16 sun6i_dsi_crc_compute(u8 const *buffer, size_t len)
 	return crc_ccitt(0xffff, buffer, len);
 }
 
-static u16 sun6i_dsi_crc_repeat_compute(u8 pd, size_t len)
+static u16 sun6i_dsi_crc_repeat(u8 pd, u8 *buffer, size_t len)
 {
-	u8 buffer[len];
-
 	memset(buffer, pd, len);
 
 	return sun6i_dsi_crc_compute(buffer, len);
@@ -274,11 +273,11 @@ static u32 sun6i_dsi_build_blk0_pkt(u8 vc, u16 wc)
 					wc & 0xff, wc >> 8);
 }
 
-static u32 sun6i_dsi_build_blk1_pkt(u16 pd, size_t len)
+static u32 sun6i_dsi_build_blk1_pkt(u16 pd, u8 *buffer, size_t len)
 {
 	u32 val = SUN6I_DSI_BLK_PD(pd);
 
-	return val | SUN6I_DSI_BLK_PF(sun6i_dsi_crc_repeat_compute(pd, len));
+	return val | SUN6I_DSI_BLK_PF(sun6i_dsi_crc_repeat(pd, buffer, len));
 }
 
 static void sun6i_dsi_inst_abort(struct sun6i_dsi *dsi)
@@ -452,6 +451,54 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 	struct mipi_dsi_device *device = dsi->device;
 	unsigned int Bpp = mipi_dsi_pixel_format_to_bpp(device->format) / 8;
 	u16 hbp, hfp, hsa, hblk, vblk;
+	size_t bytes;
+	u8 *buffer;
+
+	/* Do all timing calculations up front to allocate buffer space */
+
+	/*
+	 * A sync period is composed of a blanking packet (4 bytes +
+	 * payload + 2 bytes) and a sync event packet (4 bytes). Its
+	 * minimal size is therefore 10 bytes
+	 */
+#define HSA_PACKET_OVERHEAD	10
+	hsa = max((unsigned int)HSA_PACKET_OVERHEAD,
+		  (mode->hsync_end - mode->hsync_start) * Bpp - HSA_PACKET_OVERHEAD);
+
+	/*
+	 * The backporch is set using a blanking packet (4 bytes +
+	 * payload + 2 bytes). Its minimal size is therefore 6 bytes
+	 */
+#define HBP_PACKET_OVERHEAD	6
+	hbp = max((unsigned int)HBP_PACKET_OVERHEAD,
+		  (mode->hsync_start - mode->hdisplay) * Bpp - HBP_PACKET_OVERHEAD);
+
+	/*
+	 * The frontporch is set using a blanking packet (4 bytes +
+	 * payload + 2 bytes). Its minimal size is therefore 6 bytes
+	 */
+#define HFP_PACKET_OVERHEAD	6
+	hfp = max((unsigned int)HFP_PACKET_OVERHEAD,
+		  (mode->htotal - mode->hsync_end) * Bpp - HFP_PACKET_OVERHEAD);
+
+	/*
+	 * hblk seems to be the line + porches length.
+	 */
+	hblk = mode->htotal * Bpp - hsa;
+
+	/*
+	 * And I'm not entirely sure what vblk is about. The driver in
+	 * Allwinner BSP is using a rather convoluted calculation
+	 * there only for 4 lanes. However, using 0 (the !4 lanes
+	 * case) even with a 4 lanes screen seems to work...
+	 */
+	vblk = 0;
+
+	/* How many bytes do we need to send all payloads? */
+	bytes = max_t(size_t, max(max(hfp, hblk), max(hsa, hbp)), vblk);
+	buffer = kmalloc(bytes, GFP_KERNEL);
+	if (WARN_ON(!buffer))
+		return;
 
 	regmap_write(dsi->regs, SUN6I_DSI_BASIC_CTL_REG, 0);
 
@@ -485,63 +532,37 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 		     SUN6I_DSI_BASIC_SIZE1_VACT(mode->vdisplay) |
 		     SUN6I_DSI_BASIC_SIZE1_VT(mode->vtotal));
 
-	/*
-	 * A sync period is composed of a blanking packet (4 bytes +
-	 * payload + 2 bytes) and a sync event packet (4 bytes). Its
-	 * minimal size is therefore 10 bytes
-	 */
-#define HSA_PACKET_OVERHEAD	10
-	hsa = max((unsigned int)HSA_PACKET_OVERHEAD,
-		  (mode->hsync_end - mode->hsync_start) * Bpp - HSA_PACKET_OVERHEAD);
+	/* sync */
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HSA0_REG,
 		     sun6i_dsi_build_blk0_pkt(device->channel, hsa));
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HSA1_REG,
-		     sun6i_dsi_build_blk1_pkt(0, hsa));
+		     sun6i_dsi_build_blk1_pkt(0, buffer, hsa));
 
-	/*
-	 * The backporch is set using a blanking packet (4 bytes +
-	 * payload + 2 bytes). Its minimal size is therefore 6 bytes
-	 */
-#define HBP_PACKET_OVERHEAD	6
-	hbp = max((unsigned int)HBP_PACKET_OVERHEAD,
-		  (mode->hsync_start - mode->hdisplay) * Bpp - HBP_PACKET_OVERHEAD);
+	/* backporch */
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HBP0_REG,
 		     sun6i_dsi_build_blk0_pkt(device->channel, hbp));
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HBP1_REG,
-		     sun6i_dsi_build_blk1_pkt(0, hbp));
+		     sun6i_dsi_build_blk1_pkt(0, buffer, hbp));
 
-	/*
-	 * The frontporch is set using a blanking packet (4 bytes +
-	 * payload + 2 bytes). Its minimal size is therefore 6 bytes
-	 */
-#define HFP_PACKET_OVERHEAD	6
-	hfp = max((unsigned int)HFP_PACKET_OVERHEAD,
-		  (mode->htotal - mode->hsync_end) * Bpp - HFP_PACKET_OVERHEAD);
+	/* frontporch */
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HFP0_REG,
 		     sun6i_dsi_build_blk0_pkt(device->channel, hfp));
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HFP1_REG,
-		     sun6i_dsi_build_blk1_pkt(0, hfp));
+		     sun6i_dsi_build_blk1_pkt(0, buffer, hfp));
 
-	/*
-	 * hblk seems to be the line + porches length.
-	 */
-	hblk = mode->htotal * Bpp - hsa;
+	/* hblk */
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HBLK0_REG,
 		     sun6i_dsi_build_blk0_pkt(device->channel, hblk));
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_HBLK1_REG,
-		     sun6i_dsi_build_blk1_pkt(0, hblk));
+		     sun6i_dsi_build_blk1_pkt(0, buffer, hblk));
 
-	/*
-	 * And I'm not entirely sure what vblk is about. The driver in
-	 * Allwinner BSP is using a rather convoluted calculation
-	 * there only for 4 lanes. However, using 0 (the !4 lanes
-	 * case) even with a 4 lanes screen seems to work...
-	 */
-	vblk = 0;
+	/* vblk */
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_VBLK0_REG,
 		     sun6i_dsi_build_blk0_pkt(device->channel, vblk));
 	regmap_write(dsi->regs, SUN6I_DSI_BLK_VBLK1_REG,
-		     sun6i_dsi_build_blk1_pkt(0, vblk));
+		     sun6i_dsi_build_blk1_pkt(0, buffer, vblk));
+
+	kfree(buffer);
 }
 
 static int sun6i_dsi_start(struct sun6i_dsi *dsi,