video: mxsfb: Fix framebuffer corruption on mx6sx

Allocate the framebuffer memory as coherent, otherwise the framebuffer
will suffer from artifacts when displaying scrolling text or video.
This can be replicated on i.MX6SX (armv7), which has more complex memory
architecture compared to the i.MX23/28 (armv5).

Signed-off-by: Marek Vasut <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: Fabio Estevam <[e-mail]>
Cc: Lucas Stach <[e-mail]>
Cc: Shawn Guo <[e-mail]>
Signed-off-by: Tomi Valkeinen <[e-mail]>

drivers/video/fbdev/mxsfb.c

···
 			struct fb_videomode *vmode)
 {
 	int ret;
+	struct device *dev = &host->pdev->dev;
 	struct fb_info *fb_info = &host->fb_info;
 	struct fb_var_screeninfo *var = &fb_info->var;
 	dma_addr_t fb_phys;
···
 
 	/* Memory allocation for framebuffer */
 	fb_size = SZ_2M;
-	fb_virt = alloc_pages_exact(fb_size, GFP_DMA);
+	fb_virt = dma_alloc_wc(dev, PAGE_ALIGN(fb_size), &fb_phys, GFP_KERNEL);
 	if (!fb_virt)
 		return -ENOMEM;
-
-	fb_phys = virt_to_phys(fb_virt);
 
 	fb_info->fix.smem_start = fb_phys;
 	fb_info->screen_base = fb_virt;
···
 
 static void mxsfb_free_videomem(struct mxsfb_info *host)
 {
+	struct device *dev = &host->pdev->dev;
 	struct fb_info *fb_info = &host->fb_info;
 
-	free_pages_exact(fb_info->screen_base, fb_info->fix.smem_len);
+	dma_free_wc(dev, fb_info->screen_size, fb_info->screen_base,
+		    fb_info->fix.smem_start);
 }
 
 static const struct platform_device_id mxsfb_devtype[] = {