fb: add rudimentary drm fb and page api

This implementation uses a page flipping thread and very simple
pthreads mutex/condition variable synchronization.  There is room
for improvement, but this combined with 3 fb pages enables the
rendering loop to fully overlap with page flipping with the next
ready page queued asynchronously.

2 files changed, 380 insertions, 0 deletions

diff --git a/fb.c b/fb.c
new file mode 100644
index 0000000..97d0a17
--- /dev/null
+++ b/fb.c
@@ -0,0 +1,343 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <inttypes.h>
+#include <xf86drm.h>
+#include <xf86drmMode.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+
+#include "fb.h"
+#include "util.h"
+
+/* Copyright (C) 2016 Vito Caputo <vcaputo@pengaru.com> */
+
+
+/* I've used a separate thread for page-flipping duties because the libdrm api
+ * (and related kernel ioctl) for page flips doesn't appear to support queueing
+ * multiple flip requests.  In this use case we aren't interactive and wish to
+ * just accumulate rendered pages until we run out of spare pages, allowing the
+ * renderer to get as far ahead of vsync as possible, and certainly never
+ * blocked waiting for vsync unless there's no spare page available for drawing
+ * into.
+ *
+ * In lieu of a queueing mechanism on the drm fd, we must submit the next page
+ * once the currently submitted page is flipped to - it's at that moment we
+ * won't get EBUSY from the ioctl any longer.  Without a dedicated thread
+ * submitting flip requests and synchronously consuming their flip events,
+ * we're liable to introduce latency in the page flip submission if implemented
+ * in a more opportunistic manner whenever the fb api is entered from the
+ * render loop.
+ *
+ * If the kernel simply let us queue multiple flip requests we could maintain
+ * our submission queue entirely in the drm fd, and get available pages from
+ * the drm event handler once our pool of pages is depleted.  The kernel on
+ * vsync could check the fd to see if another flip is queued and there would be
+ * the least latency possible in submitting the flips - the least likely to
+ * miss a vsync.  This would also elide the need for synchronization in
+ * userspace between the renderer and the flipper thread, since there would no
+ * longer be a flipper thread.
+ *
+ * Let me know if you're aware of a better way with existing mainline drm!
+ */
+
+
+/* Most of fb_page_t is kept private, the public part is
+ * just an fb_fragment_t describing the whole page.
+ */
+typedef struct _fb_page_t _fb_page_t;
+struct _fb_page_t {
+	_fb_page_t	*next;
+	uint32_t	*mmap;
+	size_t		mmap_size;
+	uint32_t	drm_dumb_handle;
+	uint32_t	drm_fb_id;
+	fb_page_t	public_page;
+};
+
+typedef struct fb_t {
+	pthread_t	thread;
+	int		drm_fd;
+	uint32_t	drm_crtc_id;
+
+	_fb_page_t	*active_page;		/* page currently displayed */
+
+	pthread_mutex_t	ready_mutex;
+	pthread_cond_t	ready_cond;
+	_fb_page_t	*ready_pages_head;	/* next pages to flip to */
+	_fb_page_t	*ready_pages_tail;
+
+	pthread_mutex_t	inactive_mutex;
+	pthread_cond_t	inactive_cond;
+	_fb_page_t	*inactive_pages;	/* finished pages available for (re)use */
+
+	unsigned	put_pages_count;
+} fb_t;
+
+#ifndef container_of
+#define container_of(_ptr, _type, _member) \
+	(_type *)((void *)(_ptr) - offsetof(_type, _member))
+#endif
+
+
+/* Synchronize with the page flip by waiting for its event. */
+static void fb_drm_flip_wait(fb_t *fb)
+{
+	drmEventContext	drm_ev_ctx = {
+				.version = DRM_EVENT_CONTEXT_VERSION,
+				.vblank_handler = NULL,
+				.page_flip_handler = NULL
+			};
+	drmHandleEvent(fb->drm_fd, &drm_ev_ctx);
+}
+
+
+/* Consumes ready pages queued via fb_page_put(), submits them to drm to flip
+ * on vsync.  Produces inactive pages from those replaced, making them
+ * available to fb_page_get(). */
+static void * fb_flipper_thread(void *_fb)
+{
+	fb_t	*fb = _fb;
+
+	for (;;) {
+		_fb_page_t	*next_active_page;
+		/* wait for a flip req, submit the req page for flip on vsync, wait for it to flip before making the
+		 * active page inactive/available, repeat.
+		 */
+		pthread_mutex_lock(&fb->ready_mutex);
+		while (!fb->ready_pages_head)
+			pthread_cond_wait(&fb->ready_cond, &fb->ready_mutex);
+
+		next_active_page = fb->ready_pages_head;
+		fb->ready_pages_head = next_active_page->next;
+		if (!fb->ready_pages_head)
+			fb->ready_pages_tail = NULL;
+		pthread_mutex_unlock(&fb->ready_mutex);
+
+		/* submit the next active page for page flip on vsync, and wait for it. */
+		pexit_if(drmModePageFlip(fb->drm_fd, fb->drm_crtc_id, next_active_page->drm_fb_id, DRM_MODE_PAGE_FLIP_EVENT, NULL) < 0,
+			"unable to flip page");
+		fb_drm_flip_wait(fb);
+
+		/* now that we're displaying a new page, make the previously active one inactive so rendering can reuse it */
+		pthread_mutex_lock(&fb->inactive_mutex);
+		fb->active_page->next = fb->inactive_pages;
+		fb->inactive_pages = fb->active_page;
+		pthread_cond_signal(&fb->inactive_cond);
+		pthread_mutex_unlock(&fb->inactive_mutex);
+
+		fb->active_page = next_active_page;
+	}
+}
+
+
+/* creates a framebuffer page, which is a coupled drm_fb object and mmap region of memory */
+static void fb_page_new(fb_t *fb, drmModeModeInfoPtr mode)
+{
+	_fb_page_t			*page;
+	struct drm_mode_create_dumb	create_dumb = {
+						.width = mode->hdisplay,
+						.height = mode->vdisplay,
+						.bpp = 32,
+						.flags = 0, // unused,
+					};
+	struct drm_mode_map_dumb	map_dumb = {
+						.pad = 0, // unused
+					};
+	uint32_t			*map, fb_id;
+
+	page = calloc(1, sizeof(_fb_page_t));
+
+	pexit_if(ioctl(fb->drm_fd, DRM_IOCTL_MODE_CREATE_DUMB, &create_dumb) < 0,
+		"unable to create dumb buffer");
+
+	map_dumb.handle = create_dumb.handle;
+	pexit_if(ioctl(fb->drm_fd, DRM_IOCTL_MODE_MAP_DUMB, &map_dumb) < 0,
+		"unable to prepare dumb buffer for mmap");
+	pexit_if(!(map = mmap(NULL, create_dumb.size, PROT_READ|PROT_WRITE, MAP_SHARED, fb->drm_fd, map_dumb.offset)),
+		"unable to mmap dumb buffer");
+	pexit_if(drmModeAddFB(fb->drm_fd, mode->hdisplay, mode->vdisplay, 24, 32, create_dumb.pitch, create_dumb.handle, &fb_id) < 0,
+		"unable to add dumb buffer");
+
+	page->mmap = map;
+	page->mmap_size = create_dumb.size;
+	page->drm_dumb_handle = map_dumb.handle;
+	page->drm_fb_id = fb_id;
+
+	page->public_page.fragment.buf = map;
+	page->public_page.fragment.width = mode->hdisplay;
+	page->public_page.fragment.height = mode->vdisplay;
+	page->public_page.fragment.stride = create_dumb.pitch - (mode->hdisplay * 4);
+
+	page->next = fb->inactive_pages;
+	fb->inactive_pages = page;
+}
+
+
+static void _fb_page_free(fb_t *fb, _fb_page_t *page)
+{
+	struct drm_mode_destroy_dumb	destroy_dumb = {
+						.handle = page->drm_dumb_handle,
+					};
+
+	drmModeRmFB(fb->drm_fd, page->drm_fb_id);
+	munmap(page->mmap, page->mmap_size);
+	ioctl(fb->drm_fd, DRM_IOCTL_MODE_DESTROY_DUMB, &destroy_dumb); // XXX: errors?
+	free(page);
+}
+
+
+/* get the next inactive page from the fb, waiting if necessary. */
+static inline _fb_page_t * _fb_page_get(fb_t *fb)
+{
+	_fb_page_t	*page;
+
+	/* As long as n_pages is >= 3 this won't block unless we're submitting
+	 * pages faster than vhz.
+	 */
+	pthread_mutex_lock(&fb->inactive_mutex);
+	while (!(page = fb->inactive_pages))
+		pthread_cond_wait(&fb->inactive_cond, &fb->inactive_mutex);
+	fb->inactive_pages = page->next;
+	pthread_mutex_unlock(&fb->inactive_mutex);
+
+	page->next = NULL;
+
+	return page;
+}
+
+
+/* public interface */
+fb_page_t * fb_page_get(fb_t *fb)
+{
+	return &(_fb_page_get(fb)->public_page);
+}
+
+
+/* put a page into the fb, queueing for display */
+static inline void _fb_page_put(fb_t *fb, _fb_page_t *page)
+{
+	pthread_mutex_lock(&fb->ready_mutex);
+	if (fb->ready_pages_tail)
+		fb->ready_pages_tail->next = page;
+	else
+		fb->ready_pages_head = page;
+
+	fb->ready_pages_tail = page;
+	pthread_cond_signal(&fb->ready_cond);
+	pthread_mutex_unlock(&fb->ready_mutex);
+}
+
+
+/* public interface */
+
+/* put a page into the fb, queueing for display */
+void fb_page_put(fb_t *fb, fb_page_t *page)
+{
+	fb->put_pages_count++;
+
+	_fb_page_put(fb, container_of(page, _fb_page_t, public_page));
+}
+
+
+/* get (and reset) the current count of put pages */
+void fb_get_put_pages_count(fb_t *fb, unsigned *count)
+{
+	*count = fb->put_pages_count;
+	fb->put_pages_count = 0;
+}
+
+
+/* free the fb and associated resources */
+void fb_free(fb_t *fb)
+{
+	if (fb->active_page) {
+		pthread_cancel(fb->thread);
+		pthread_join(fb->thread, NULL);
+	}
+
+	/* TODO: free all the pages */
+
+	pthread_mutex_destroy(&fb->ready_mutex);
+	pthread_cond_destroy(&fb->ready_cond);
+	pthread_mutex_destroy(&fb->inactive_mutex);
+	pthread_cond_destroy(&fb->inactive_cond);
+
+	free(fb);
+}
+
+
+/* create a new fb instance */
+fb_t * fb_new(int drm_fd, uint32_t crtc_id, uint32_t *connectors, int n_connectors, drmModeModeInfoPtr mode, int n_pages)
+{
+	int		i;
+	_fb_page_t	*page;
+	fb_t		*fb;
+
+	fb = calloc(1, sizeof(fb_t));
+	if (!fb)
+		return NULL;
+
+	fb->drm_fd = drm_fd;
+	fb->drm_crtc_id = crtc_id;
+
+	for (i = 0; i < n_pages; i++)
+		fb_page_new(fb, mode);
+
+	pthread_mutex_init(&fb->ready_mutex, NULL);
+	pthread_cond_init(&fb->ready_cond, NULL);
+	pthread_mutex_init(&fb->inactive_mutex, NULL);
+	pthread_cond_init(&fb->inactive_cond, NULL);
+
+	page = _fb_page_get(fb);
+
+	/* set the video mode, pinning this page, set it as the active page. */
+	if (drmModeSetCrtc(drm_fd, crtc_id, page->drm_fb_id, 0, 0, connectors, n_connectors, mode) < 0) {
+		_fb_page_free(fb, page);
+		fb_free(fb);
+		return NULL;
+	}
+
+	fb->active_page = page;
+
+	/* start up the page flipper thread */
+	pthread_create(&fb->thread, NULL, fb_flipper_thread, fb);
+
+	return fb;
+}
+
+
+/* divide a fragment into n_fragments, storing their values into fragments[],
+ * which is expected to have n_fragments of space. */
+void fb_fragment_divide(fb_fragment_t *fragment, unsigned n_fragments, fb_fragment_t fragments[])
+{
+	unsigned	slice = fragment->height / n_fragments;
+	unsigned	i;
+	void		*buf = fragment->buf;
+	unsigned	pitch = (fragment->width * 4) + fragment->stride;
+	unsigned	y = fragment->y;
+
+	/* This just splits the supplied fragment into even horizontal slices */
+	/* TODO: It probably makes sense to add an fb_fragment_tile() as well, since some rendering
+	 * algorithms benefit from the locality of a tiled fragment.
+	 */
+
+	for (i = 0; i < n_fragments; i++) {
+		fragments[i].buf = buf;
+		fragments[i].x = fragment->x;
+		fragments[i].y = y;
+		fragments[i].width = fragment->width;
+		fragments[i].height = slice;
+		fragments[i].stride = fragment->stride;
+
+		buf += pitch * slice;
+		y += slice;
+	}
+	/* TODO: handle potential fractional tail slice? */
+}
diff --git a/fb.h b/fb.h
new file mode 100644
index 0000000..13f6bcd
--- /dev/null
+++ b/fb.h
@@ -0,0 +1,37 @@
+#ifndef _FB_H
+#define _FB_H
+
+#include <stdint.h>
+#include <sys/types.h>
+#include <xf86drmMode.h> /* for drmModeModeInfoPtr */
+
+/* All renderers should target fb_fragment_t, which may or may not represent
+ * a full-screen mmap.  Helpers are provided for subdividing fragments for
+ * concurrent renderers.
+ */
+typedef struct fb_fragment_t {
+	uint32_t	*buf;		/* pointer to the first pixel in the fragment */
+	unsigned	x, y;		/* absolute coordinates of the upper left corner of this fragment */
+	unsigned	width, height;	/* width and height of this fragment */
+	unsigned	stride;		/* number of bytes from the end of one row to the start of the next */
+} fb_fragment_t;
+
+/* This is a page handle object for page flip submission/life-cycle.
+ * Outside of fb_page_get()/fb_page_put(), you're going to be interested in
+ * fb_fragment_t.  The fragment included here describes the whole page,
+ * it may be divided via fb_fragment_divide().
+ */
+typedef struct fb_page_t {
+	fb_fragment_t	fragment;
+} fb_page_t;
+
+typedef struct fb_t fb_t;
+
+fb_page_t * fb_page_get(fb_t *fb);
+void fb_page_put(fb_t *fb, fb_page_t *page);
+void fb_free(fb_t *fb);
+void fb_get_put_pages_count(fb_t *fb, unsigned *count);
+fb_t * fb_new(int drm_fd, uint32_t crtc_id, uint32_t *connectors, int n_connectors, drmModeModeInfoPtr mode, int n_pages);
+void fb_fragment_divide(fb_fragment_t *fragment, unsigned n_fragments, fb_fragment_t fragments[]);
+
+#endif