[vram] Add "vram" built-in setting to dump video RAM

The "vram" setting returns the (Base64-encoded) contents of video RAM,
and can be used to capture a screenshot.  For example: after running
memtest.0 and encountering an error, the output can be captured and
sent to a remote server for later diagnosis:

  #!ipxe
  chain -a http://server/memtest.0 && goto ok || goto bad
  :bad
  params
  param errno ${errno}
  param vram ${vram}
  chain -a http://server/report.php##params
  :ok

Inspired-by: Christian Nilsson <nikize@gmail.com>
Originally-implemented-by: Christian Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/core/vram_settings.c

+/*
+ * Copyright (C) 2015 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/uaccess.h>
+#include <ipxe/settings.h>
+
+/** @file
+ *
+ * Video RAM dump
+ *
+ */
+
+/** Video RAM base address */
+#define VRAM_BASE 0xb8000
+
+/** Video RAM length */
+#define VRAM_LEN \
+	( 80 /* columns */ * 25 /* rows */ * 2 /* bytes per character */ )
+
+/**
+ * Fetch video RAM setting
+ *
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
+ */
+static int vram_fetch ( void *data, size_t len ) {
+	userptr_t vram = phys_to_user ( VRAM_BASE );
+
+	/* Copy video RAM */
+	if ( len > VRAM_LEN )
+		len = VRAM_LEN;
+	copy_from_user ( data, vram, 0, len );
+
+	return VRAM_LEN;
+}
+
+/** Video RAM setting */
+const struct setting vram_setting __setting ( SETTING_MISC, vram ) = {
+	.name = "vram",
+	.description = "Video RAM",
+	.type = &setting_type_base64,
+	.scope = &builtin_scope,
+};
+
+/** Video RAM built-in setting */
+struct builtin_setting vram_builtin_setting __builtin_setting = {
+	.setting = &vram_setting,
+	.fetch = vram_fetch,
+};

src/config/config.c

 #ifdef MEMMAP_SETTINGS
 REQUIRE_OBJECT ( memmap_settings );
 #endif
+#ifdef VRAM_SETTINGS
+REQUIRE_OBJECT ( vram_settings );
+#endif
 
 /*
  * Drag in selected keyboard map

src/config/settings.h

 //#define	CPUID_SETTINGS	/* CPUID settings */
 //#define	MEMMAP_SETTINGS	/* Memory map settings */
 //#define	VMWARE_SETTINGS	/* VMware GuestInfo settings */
+//#define	VRAM_SETTINGS	/* Video RAM dump settings */
 
 #include <config/named.h>
 #include NAMED_CONFIG(settings.h)