Update HWC documentation for virtual displays
diff --git a/include/hardware/hwcomposer.h b/include/hardware/hwcomposer.h
index 846bab4..86479d3 100644
@@ -315,8 +315,8 @@
- /* WARNING: These fields are for experimental virtual display support,
- * and are not currently used. */
+ /* These fields are used for virtual displays when the h/w composer
+ * version is at least HWC_DEVICE_VERSION_1_3. */
/* outbuf is the buffer that receives the composed image for
* virtual displays. Writes to the outbuf must wait until
@@ -324,14 +324,28 @@
* writes to outbuf are complete should be returned in
- * This field will not be updated until after prepare(). If
- * prepare() sets all non-FB layers to OVERLAY or sets all non-FB
- * layers to FRAMEBUFFER, then the FRAMEBUFFER_TARGET buffer and
- * the output buffer may be the same. In mixed OVERLAY/FRAMEBUFFER
- * configurations they will have different buffers so the
- * h/w composer does not have to read and write the same buffer.
+ * This field is set before prepare(), so properties of the buffer
+ * can be used to decide which layers can be handled by h/w
+ * composer.
- * For physical displays, outbuf will be NULL.
+ * If prepare() sets all layers to FRAMEBUFFER, then GLES
+ * composition will happen directly to the output buffer. In this
+ * case, both outbuf and the FRAMEBUFFER_TARGET layer's buffer will
+ * be the same, and set() has no work to do besides managing fences.
+ * If the TARGET_FORCE_HWC_FOR_VIRTUAL_DISPLAYS board config
+ * variable is defined (not the default), then this behavior is
+ * changed: if all layers are marked for FRAMEBUFFER, GLES
+ * composition will take place to a scratch framebuffer, and
+ * h/w composer must copy it to the output buffer. This allows the
+ * h/w composer to do format conversion if there are cases where
+ * that is more desirable than doing it in the GLES driver or at the
+ * virtual display consumer.
+ * If some or all layers are marked OVERLAY, then the framebuffer
+ * and output buffer will be different. As with physical displays,
+ * the framebuffer handle will not change between frames if all
+ * layers are marked for OVERLAY.