drm/fb-helper: Map DRM client buffer only when required

This patch changes DRM clients to not map the buffer by default. The
buffer, like any buffer object, should be mapped and unmapped when
needed.

An unmapped buffer object can be evicted to system memory and does
not consume video ram until displayed. This allows to use generic fbdev
emulation with drivers for low-memory devices, such as ast and mgag200.

This change affects the generic framebuffer console. HW-based consoles
map their console buffer once and keep it mapped. Userspace can mmap this
buffer into its address space. The shadow-buffered framebuffer console
only needs the buffer object to be mapped during updates. While not being
updated from the shadow buffer, the buffer object can remain unmapped.
Userspace will always mmap the shadow buffer.

v2:
* change DRM client to not map buffer by default
* manually map client buffer for fbdev with HW framebuffer

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Noralf Trønnes <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/315830/
Signed-off-by: Gerd Hoffmann <[e-mail]>

drivers/gpu/drm/drm_client.c

···
 	struct drm_device *dev = client->dev;
 	struct drm_client_buffer *buffer;
 	struct drm_gem_object *obj;
-	void *vaddr;
 	int ret;
 
 	buffer = kzalloc(sizeof(*buffer), GFP_KERNEL);
···
 
 	buffer->gem = obj;
 
-	vaddr = drm_client_buffer_vmap(buffer);
-	if (IS_ERR(vaddr)) {
-		ret = PTR_ERR(vaddr);
-		goto err_delete;
-	}
-
 	return buffer;
 
 err_delete:
···
  * Client buffer mappings are not ref'counted. Each call to
  * drm_client_buffer_vmap() should be followed by a call to
  * drm_client_buffer_vunmap(); or the client buffer should be mapped
- * throughout its lifetime. The latter is the default.
+ * throughout its lifetime.
  *
  * Returns:
  *	The mapped memory's address
···
  * drm_client_buffer_vunmap - Unmap DRM client buffer
  * @buffer: DRM client buffer
  *
- * This function removes a client buffer's memory mapping. This
- * function is only required by clients that manage their buffers
- * by themselves. By default, DRM client buffers are mapped throughout
- * their entire lifetime.
+ * This function removes a client buffer's memory mapping. Calling this
+ * function is only required by clients that manage their buffer mappings
+ * by themselves.
  */
 void drm_client_buffer_vunmap(struct drm_client_buffer *buffer)
 {

drivers/gpu/drm/drm_fb_helper.c

···
 	struct drm_clip_rect *clip = &helper->dirty_clip;
 	struct drm_clip_rect clip_copy;
 	unsigned long flags;
+	void *vaddr;
 
 	spin_lock_irqsave(&helper->dirty_lock, flags);
 	clip_copy = *clip;
···
 
 	/* call dirty callback only when it has been really touched */
 	if (clip_copy.x1 < clip_copy.x2 && clip_copy.y1 < clip_copy.y2) {
+
 		/* Generic fbdev uses a shadow buffer */
-		if (helper->buffer)
+		if (helper->buffer) {
+			vaddr = drm_client_buffer_vmap(helper->buffer);
+			if (IS_ERR(vaddr))
+				return;
 			drm_fb_helper_dirty_blit_real(helper, &clip_copy);
+		}
 		helper->fb->funcs->dirty(helper->fb, NULL, 0, 0, &clip_copy, 1);
+
+		if (helper->buffer)
+			drm_client_buffer_vunmap(helper->buffer);
 	}
 }
 
···
 	struct drm_framebuffer *fb;
 	struct fb_info *fbi;
 	u32 format;
+	void *vaddr;
 
 	DRM_DEBUG_KMS("surface width(%d), height(%d) and bpp(%d)\n",
 		      sizes->surface_width, sizes->surface_height,
···
 	fbi->fbops = &drm_fbdev_fb_ops;
 	fbi->screen_size = fb->height * fb->pitches[0];
 	fbi->fix.smem_len = fbi->screen_size;
-	fbi->screen_buffer = buffer->vaddr;
-	/* Shamelessly leak the physical address to user-space */
-#if IS_ENABLED(CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM)
-	if (drm_leak_fbdev_smem && fbi->fix.smem_start == 0)
-		fbi->fix.smem_start =
-			page_to_phys(virt_to_page(fbi->screen_buffer));
-#endif
+
 	drm_fb_helper_fill_info(fbi, fb_helper, sizes);
 
 	if (fb->funcs->dirty) {
···
 		fbi->fbdefio = &drm_fbdev_defio;
 
 		fb_deferred_io_init(fbi);
+	} else {
+		/* buffer is mapped for HW framebuffer */
+		vaddr = drm_client_buffer_vmap(fb_helper->buffer);
+		if (IS_ERR(vaddr))
+			return PTR_ERR(vaddr);
+
+		fbi->screen_buffer = vaddr;
+		/* Shamelessly leak the physical address to user-space */
+#if IS_ENABLED(CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM)
+		if (drm_leak_fbdev_smem && fbi->fix.smem_start == 0)
+			fbi->fix.smem_start =
+				page_to_phys(virt_to_page(fbi->screen_buffer));
+#endif
 	}
 
 	return 0;