fbtft: Add a sysfs parameter to set the vertical scrolling position on a window.

Add a sysfs param scroll_pos which sets the scanline where the display
starts for 'rolling scrolling'. This needs support from the specific
display driver as well. When supported it can be used to implement smooth
vertical (when the display is in portrait mode) hor horizontal (when the
display is used in landscape mode) scrolling by strategically changing the
start scanline of the display and erasing/redrawing only the wrapping part.
This way you never need to redraw what's already there, preventing slow
full screen redraws over spi.

Author: Martijn Versteegh

drivers/staging/fbtft/fbtft-core.c

@@ -623,6 +623,9 @@ static void fbtft_merge_fbtftops(struct fbtft_ops *dst, struct fbtft_ops *src)
 		dst->set_var = src->set_var;
 	if (src->set_gamma)
 		dst->set_gamma = src->set_gamma;
+	if (src->set_scroll)
+		dst->set_scroll = src->set_scroll;
+
 }
 
 /**

drivers/staging/fbtft/fbtft-sysfs.c

@@ -204,16 +204,55 @@ static ssize_t show_debug(struct device *device,
 static struct device_attribute debug_device_attr = \
 	__ATTR(debug, 0660, show_debug, store_debug);
 
+
+
+static ssize_t store_scroll(struct device *device,
+				struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct fb_info *fb_info = dev_get_drvdata(device);
+	struct fbtft_par *par = fb_info->par;
+	int ret;
+
+	ret = kstrtoul(buf, 10, &par->scroll_pos);
+
+	if (ret)
+		return ret;
+
+	if (par->fbtftops.set_scroll)
+		par->fbtftops.set_scroll(par);
+
+
+	return count;
+}
+
+static ssize_t show_scroll(struct device *device,
+				struct device_attribute *attr, char *buf)
+{
+	struct fb_info *fb_info = dev_get_drvdata(device);
+	struct fbtft_par *par = fb_info->par;
+
+	return snprintf(buf, PAGE_SIZE, "%lu\n", par->scroll_pos);
+}
+
+static struct device_attribute scroll_pos_device_attr = \
+	__ATTR(scroll_pos, 0660, show_scroll, store_scroll);
+
+
 void fbtft_sysfs_init(struct fbtft_par *par)
 {
 	device_create_file(par->info->dev, &debug_device_attr);
+	if (par->fbtftops.set_scroll)
+		device_create_file(par->info->dev, &scroll_pos_device_attr);
 	if (par->gamma.curves && par->fbtftops.set_gamma)
 		device_create_file(par->info->dev, &gamma_device_attrs[0]);
 }
 
 void fbtft_sysfs_exit(struct fbtft_par *par)
 {
 	device_remove_file(par->info->dev, &debug_device_attr);
+	if (par->fbtftops.set_scroll)
+		device_remove_file(par->info->dev, &scroll_pos_device_attr);
 	if (par->gamma.curves && par->fbtftops.set_gamma)
 		device_remove_file(par->info->dev, &gamma_device_attrs[0]);
 }

drivers/staging/fbtft/fbtft.h

@@ -93,6 +93,7 @@ struct fbtft_ops {
 
 	int (*set_var)(struct fbtft_par *par);
 	int (*set_gamma)(struct fbtft_par *par, unsigned long *curves);
+	void (*set_scroll)(struct fbtft_par *par);
 };
 
 /**
@@ -240,6 +241,7 @@ struct fbtft_par {
 	bool first_update_done;
 	ktime_t update_time;
 	bool bgr;
+	unsigned long scroll_pos;
 	void *extra;
 };