[vesafb] Allow for an arbitrary margin around the text area

Allow for an arbitrary margin to be specified in the console
configuration.  If the actual screen size does not match the requested
screen size, then update any margins specified so that they remain in
the same place relative to the requested screen size.  If margins are
unspecified (i.e. zero), then leave them as zero.

The underlying assumption here is that any specified margins are
likely to describe an area within a background picture, and so should
remain in the same place relative to that background picture.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/vesafb.c

 /**
  * Initialise VESA frame buffer
  *
- * @v min_width		Minimum required width (in pixels)
- * @v min_height	Minimum required height (in pixels)
- * @v min_bpp		Minimum required colour depth (in bits per pixel)
- * @v pixbuf		Background picture (if any)
+ * @v config		Console configuration, or NULL to reset
  * @ret rc		Return status code
  */
-static int vesafb_init ( unsigned int min_width, unsigned int min_height,
-			 unsigned int min_bpp, struct pixel_buffer *pixbuf ) {
+static int vesafb_init ( struct console_configuration *config ) {
 	uint32_t discard_b;
 	uint16_t *mode_numbers;
+	unsigned int xgap;
+	unsigned int ygap;
+	unsigned int left;
+	unsigned int right;
+	unsigned int top;
+	unsigned int bottom;
 	int mode_number;
 	int rc;
 
 		goto err_mode_list;
 
 	/* Select mode */
-	if ( ( mode_number = vesafb_select_mode ( mode_numbers, min_width,
-						  min_height, min_bpp ) ) < 0 ){
+	if ( ( mode_number = vesafb_select_mode ( mode_numbers, config->width,
+						  config->height,
+						  config->bpp ) ) < 0 ) {
 		rc = mode_number;
 		goto err_select_mode;
 	}
 	if ( ( rc = vesafb_set_mode ( mode_number ) ) != 0 )
 		goto err_set_mode;
 
+	/* Calculate margin.  If the actual screen size is larger than
+	 * the requested screen size, then update the margins so that
+	 * the margin remains relative to the requested screen size.
+	 * (As an exception, if a zero margin was specified then treat
+	 * this as meaning "expand to edge of actual screen".)
+	 */
+	xgap = ( vesafb.pixel.width - config->width );
+	ygap = ( vesafb.pixel.height - config->height );
+	left = ( xgap / 2 );
+	right = ( xgap - left );
+	top = ( ygap / 2 );
+	bottom = ( ygap - top );
+	vesafb.margin.left = ( config->left + ( config->left ? left : 0 ) );
+	vesafb.margin.right = ( config->right + ( config->right ? right : 0 ) );
+	vesafb.margin.top = ( config->top + ( config->top ? top : 0 ) );
+	vesafb.margin.bottom =
+		( config->bottom + ( config->bottom ? bottom : 0 ) );
+
 	/* Get font data */
 	vesafb_font();
 
 	/* Initialise frame buffer console */
 	if ( ( rc = fbcon_init ( &vesafb.fbcon, phys_to_user ( vesafb.start ),
 				 &vesafb.pixel, &vesafb.margin, &vesafb.map,
-				 &vesafb.font, pixbuf ) ) != 0 )
+				 &vesafb.font, config->pixbuf ) ) != 0 )
 		goto err_fbcon_init;
 
 	free ( mode_numbers );
 	}
 
 	/* Initialise VESA frame buffer */
-	if ( ( rc = vesafb_init ( config->width, config->height, config->bpp,
-				  config->pixbuf ) ) != 0 )
+	if ( ( rc = vesafb_init ( config ) ) != 0 )
 		return rc;
 
 	/* Mark console as enabled */