drm/bochs: Add screen blanking support

On bochs DRM driver, the execution of "setterm --blank force" results
in a frozen screen instead of a blank screen.  It's due to the lack of
the screen blanking support in its code.

Actually, the QEMU bochs vga side can switch to the blanking mode when
the bit 0x20 is cleared on VGA_ATT_IW register (0x3c0), which updates
ar_index in QEMU side.  So, essentially, we'd just need to clear the
bit at pipe disable callback; that's what this patch does essentially.

However, a tricky part is that the access via VGA_ATT_IW is done in
"flip-flop"; the first write is for index and the second write is for
the data like palette.  Meanwhile, in the current bochs DRM driver,
the flip-flop wasn't considered, and it calls only the register update
once with the value 0x20.

The spec and the actual VGA implementation in QEMU suggests that the
flip flop flag is discarded by reading the CRTC index register
(VGA_IS1_RC, 0x3da).  So, in this patch, we add the helper to read a
byte and the call to clear the flip flop flag before changing the
blank / unblank setup via VGA_ATT_IW register.

v1->v2:
* discard ar_flip_flop by reading 0x3da, add bochs_vga_readb()
* include video/vga.h for VGA register definitions
* move the blank/unblank code to bochs_hw_blank()
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421080859.14869-1-tiwai@suse.de

drivers/gpu/drm/bochs/bochs.h

@@ -78,6 +78,7 @@ struct bochs_device {
 int bochs_hw_init(struct drm_device *dev);
 void bochs_hw_fini(struct drm_device *dev);
 
+void bochs_hw_blank(struct bochs_device *bochs, bool blank);
 void bochs_hw_setmode(struct bochs_device *bochs,
 		      struct drm_display_mode *mode);
 void bochs_hw_setformat(struct bochs_device *bochs,

drivers/gpu/drm/bochs/bochs_hw.c

@@ -7,6 +7,7 @@
 #include <drm/drm_drv.h>
 #include <drm/drm_fourcc.h>
 
+#include <video/vga.h>
 #include "bochs.h"
 
 /* ---------------------------------------------------------------------- */
@@ -24,6 +25,19 @@ static void bochs_vga_writeb(struct bochs_device *bochs, u16 ioport, u8 val)
 	}
 }
 
+static u8 bochs_vga_readb(struct bochs_device *bochs, u16 ioport)
+{
+	if (WARN_ON(ioport < 0x3c0 || ioport > 0x3df))
+		return 0xff;
+
+	if (bochs->mmio) {
+		int offset = ioport - 0x3c0 + 0x400;
+		return readb(bochs->mmio + offset);
+	} else {
+		return inb(ioport);
+	}
+}
+
 static u16 bochs_dispi_read(struct bochs_device *bochs, u16 reg)
 {
 	u16 ret = 0;
@@ -205,6 +219,15 @@ void bochs_hw_fini(struct drm_device *dev)
 	kfree(bochs->edid);
 }
 
+void bochs_hw_blank(struct bochs_device *bochs, bool blank)
+{
+	DRM_DEBUG_DRIVER("hw_blank %d\n", blank);
+	/* discard ar_flip_flop */
+	(void)bochs_vga_readb(bochs, VGA_IS1_RC);
+	/* blank or unblank; we need only update index and set 0x20 */
+	bochs_vga_writeb(bochs, VGA_ATT_W, blank ? 0 : 0x20);
+}
+
 void bochs_hw_setmode(struct bochs_device *bochs,
 		      struct drm_display_mode *mode)
 {
@@ -223,7 +246,7 @@ void bochs_hw_setmode(struct bochs_device *bochs,
 			 bochs->xres, bochs->yres, bochs->bpp,
 			 bochs->yres_virtual);
 
-	bochs_vga_writeb(bochs, 0x3c0, 0x20); /* unblank */
+	bochs_hw_blank(bochs, false);
 
 	bochs_dispi_write(bochs, VBE_DISPI_INDEX_ENABLE,      0);
 	bochs_dispi_write(bochs, VBE_DISPI_INDEX_BPP,         bochs->bpp);

drivers/gpu/drm/bochs/bochs_kms.c

@@ -57,6 +57,13 @@ static void bochs_pipe_enable(struct drm_simple_display_pipe *pipe,
 	bochs_plane_update(bochs, plane_state);
 }
 
+static void bochs_pipe_disable(struct drm_simple_display_pipe *pipe)
+{
+	struct bochs_device *bochs = pipe->crtc.dev->dev_private;
+
+	bochs_hw_blank(bochs, true);
+}
+
 static void bochs_pipe_update(struct drm_simple_display_pipe *pipe,
 			      struct drm_plane_state *old_state)
 {
@@ -67,6 +74,7 @@ static void bochs_pipe_update(struct drm_simple_display_pipe *pipe,
 
 static const struct drm_simple_display_pipe_funcs bochs_pipe_funcs = {
 	.enable	    = bochs_pipe_enable,
+	.disable    = bochs_pipe_disable,
 	.update	    = bochs_pipe_update,
 	.prepare_fb = drm_gem_vram_simple_display_pipe_prepare_fb,
 	.cleanup_fb = drm_gem_vram_simple_display_pipe_cleanup_fb,