video: ssd1307fb: Make use of horizontal addressing mode

By default, the ssd1307 controller uses an addressing mode called page
addressing. This mode only increments the column cursor in memory when
writing data but will not increments the page cursor when we are at the
end of the page.

However, the controller supports another addressing mode, called
horizontal addressing, that will maintain both the page and column
cursors when writing data to the controller.

That means that we can just remove the code that increments the current
page address and reset the column cursor when reaching the end of the
line, allowing to have a lower data overhead, and a simpler driver.

Signed-off-by: Maxime Ripard <[e-mail]>
Signed-off-by: Tomi Valkeinen <[e-mail]>

drivers/video/ssd1307fb.c

···
 #define SSD1307FB_DATA			0x40
 #define SSD1307FB_COMMAND		0x80
 
+#define SSD1307FB_SET_ADDRESS_MODE	0x20
+#define SSD1307FB_SET_ADDRESS_MODE_HORIZONTAL	(0x00)
+#define SSD1307FB_SET_ADDRESS_MODE_VERTICAL	(0x01)
+#define SSD1307FB_SET_ADDRESS_MODE_PAGE		(0x02)
+#define SSD1307FB_SET_COL_RANGE		0x21
+#define SSD1307FB_SET_PAGE_RANGE	0x22
 #define SSD1307FB_CONTRAST		0x81
 #define	SSD1307FB_CHARGE_PUMP		0x8d
 #define SSD1307FB_SEG_REMAP_ON		0xa1
···
 
 static void ssd1307fb_update_display(struct ssd1307fb_par *par)
 {
+	struct ssd1307fb_array *array;
 	u8 *vmem = par->info->screen_base;
 	int i, j, k;
+
+	array = ssd1307fb_alloc_array(par->width * par->height / 8,
+				      SSD1307FB_DATA);
+	if (!array)
+		return;
 
 	/*
 	 * The screen is divided in pages, each having a height of 8
···
 	 */
 
 	for (i = 0; i < (par->height / 8); i++) {
-		struct ssd1307fb_array *array;
-		ssd1307fb_write_cmd(par->client,
-				    SSD1307FB_START_PAGE_ADDRESS + i + par->page_offset);
-		ssd1307fb_write_cmd(par->client, 0x00);
-		ssd1307fb_write_cmd(par->client, 0x10);
-
-		array = ssd1307fb_alloc_array(par->width, SSD1307FB_DATA);
-
 		for (j = 0; j < par->width; j++) {
-			array->data[j] = 0;
+			u32 array_idx = i * par->width + j;
+			array->data[array_idx] = 0;
 			for (k = 0; k < 8; k++) {
 				u32 page_length = par->width * i;
 				u32 index = page_length + (par->width * k + j) / 8;
 				u8 byte = *(vmem + index);
 				u8 bit = byte & (1 << (j % 8));
 				bit = bit >> (j % 8);
-				array->data[j] |= bit << k;
+				array->data[array_idx] |= bit << k;
 			}
 		}
-
-		ssd1307fb_write_array(par->client, array, par->width);
-		kfree(array);
 	}
+
+	ssd1307fb_write_array(par->client, array, par->width * par->height / 8);
+	kfree(array);
 }
 
 
···
 	/* Turn on the DC-DC Charge Pump */
 	ret = ssd1307fb_write_cmd(par->client, SSD1307FB_CHARGE_PUMP);
 	ret = ret & ssd1307fb_write_cmd(par->client, 0x14);
+	if (ret < 0)
+		return ret;
+
+	/* Switch to horizontal addressing mode */
+	ret = ssd1307fb_write_cmd(par->client, SSD1307FB_SET_ADDRESS_MODE);
+	ret = ret & ssd1307fb_write_cmd(par->client,
+					SSD1307FB_SET_ADDRESS_MODE_HORIZONTAL);
+	if (ret < 0)
+		return ret;
+
+	ret = ssd1307fb_write_cmd(par->client, SSD1307FB_SET_COL_RANGE);
+	ret = ret & ssd1307fb_write_cmd(par->client, 0x0);
+	ret = ret & ssd1307fb_write_cmd(par->client, par->width - 1);
+	if (ret < 0)
+		return ret;
+
+	ret = ssd1307fb_write_cmd(par->client, SSD1307FB_SET_PAGE_RANGE);
+	ret = ret & ssd1307fb_write_cmd(par->client, 0x0);
+	ret = ret & ssd1307fb_write_cmd(par->client,
+					par->page_offset + (par->height / 8) - 1);
 	if (ret < 0)
 		return ret;