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File: /arch/arm/include/asm/param.h

# define HZ		CONFIG_HZ	/* Internal kernel timer frequency */

File: .config
// default in Raspian
CONFIG_HZ=100

	fbdefio->delay =           HZ/fps;

File: include/linux/fb.h

struct fb_info {
...
#ifdef CONFIG_FB_DEFERRED_IO
	struct delayed_work deferred_work;
	struct fb_deferred_io *fbdefio;
#endif
...
};

#ifdef CONFIG_FB_DEFERRED_IO
struct fb_deferred_io {
	/* delay between mkwrite and deferred handler */
	unsigned long delay;
	struct mutex lock; /* mutex that protects the page list */
	struct list_head pagelist; /* list of touched pages */
	/* callback */
	void (*first_io)(struct fb_info *info);
	void (*deferred_io)(struct fb_info *info, struct list_head *pagelist);
};
#endif

File: include/linux/workqueue.h

struct delayed_work {
        struct work_struct work;
        struct timer_list timer;

        /* target workqueue and CPU ->timer uses to queue ->work */
        struct workqueue_struct *wq;
        int cpu;
};

File: adafruit22fb.c
static int adafruit22fb_probe(struct spi_device *spi)
...
	info = fbtft_framebuffer_alloc(&adafruit22_display, &spi->dev);

File: fbtft-core.c
struct fb_info *fbtft_framebuffer_alloc(struct fbtft_display *display, struct device *dev)
...
	unsigned fps = display->fps;
...
	/* defaults */
	if (!fps)
		fps = 20;
...
	/* platform_data override ? */
	if (pdata) {
		if (pdata->fps)
			fps = pdata->fps;
...
	fbdefio = kzalloc(sizeof(struct fb_deferred_io), GFP_KERNEL);
...
	info->fbdefio = fbdefio;
...
	fbdefio->delay =           HZ/fps;
	fbdefio->deferred_io =     fbtft_deferred_io;
	fb_deferred_io_init(info);

File: drivers/video/fb_defio.c
void fb_deferred_io_init(struct fb_info *info)
{
	struct fb_deferred_io *fbdefio = info->fbdefio;

	BUG_ON(!fbdefio);
	mutex_init(&fbdefio->lock);
	info->fbops->fb_mmap = fb_deferred_io_mmap;
	INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work);
	INIT_LIST_HEAD(&fbdefio->pagelist);
	if (fbdefio->delay == 0) /* set a default of 1 s */
		fbdefio->delay = HZ;
}

File: include/linux/workqueue.h
// INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work)
#define INIT_DELAYED_WORK(_work, _func)					\
	__INIT_DELAYED_WORK(_work, _func, 0)

// __INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work, 0)
#define __INIT_DELAYED_WORK(_work, _func, _tflags)			\
	do {								\
		INIT_WORK(&(_work)->work, (_func));			\
		__setup_timer(&(_work)->timer, delayed_work_timer_fn,	\
			      (unsigned long)(_work),			\
			      (_tflags) | TIMER_IRQSAFE);		\
	} while (0)

// INIT_WORK(&info->deferred_work->work, fb_deferred_io_work);
#define INIT_WORK(_work, _func)						\
	do {								\
		__INIT_WORK((_work), (_func), 0);			\
	} while (0)

// __INIT_WORK(&info->deferred_work->work, fb_deferred_io_work, 0);
#define __INIT_WORK(_work, _func, _onstack)				\
	do {								\
		__init_work((_work), _onstack);				\
		(_work)->data = (atomic_long_t) WORK_DATA_INIT();	\
		INIT_LIST_HEAD(&(_work)->entry);			\
		PREPARE_WORK((_work), (_func));				\
	} while (0)

File: kernel/workqueue.c

// __init_work(&info->deferred_work->work, 0);
void __init_work(struct work_struct *work, int onstack)
{
	if (onstack)
		debug_object_init_on_stack(work, &work_debug_descr);
	else
		debug_object_init(work, &work_debug_descr);
}

// PREPARE_WORK(&info->deferred_work->work, fb_deferred_io_work);
/*
 * initialize a work item's function pointer
 */
#define PREPARE_WORK(_work, _func)					\
	do {								\
		(_work)->func = (_func);				\
	} while (0)

File: include/linux/timer.h

// __setup_timer(&info->deferred_work->timer, delayed_work_timer_fn, (unsigned long)(&info->deferred_work), TIMER_IRQSAFE);
#define __setup_timer(_timer, _fn, _data, _flags)			\
	do {								\
		__init_timer((_timer), (_flags));			\
		(_timer)->function = (_fn);				\
		(_timer)->data = (_data);				\
	} while (0)

File: drivers/video/fb_defio.c

static const struct vm_operations_struct fb_deferred_io_vm_ops = {
	.fault		= fb_deferred_io_fault,
	.page_mkwrite	= fb_deferred_io_mkwrite,
};

static int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
{
	vma->vm_ops = &fb_deferred_io_vm_ops;
	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
	if (!(info->flags & FBINFO_VIRTFB))
		vma->vm_flags |= VM_IO;
	vma->vm_private_data = info;
	return 0;
}

/* vm_ops->page_mkwrite handler */
static int fb_deferred_io_mkwrite(struct vm_area_struct *vma,
				  struct vm_fault *vmf)
{
	struct page *page = vmf->page;
	struct fb_info *info = vma->vm_private_data;
	struct fb_deferred_io *fbdefio = info->fbdefio;
	struct page *cur;

	/* this is a callback we get when userspace first tries to
	write to the page. we schedule a workqueue. that workqueue
	will eventually mkclean the touched pages and execute the
	deferred framebuffer IO. then if userspace touches a page
	again, we repeat the same scheme */

	file_update_time(vma->vm_file);

	/* protect against the workqueue changing the page list */
	mutex_lock(&fbdefio->lock);

	/* first write in this cycle, notify the driver */
	if (fbdefio->first_io && list_empty(&fbdefio->pagelist))
		fbdefio->first_io(info);

	/*
	 * We want the page to remain locked from ->page_mkwrite until
	 * the PTE is marked dirty to avoid page_mkclean() being called
	 * before the PTE is updated, which would leave the page ignored
	 * by defio.
	 * Do this by locking the page here and informing the caller
	 * about it with VM_FAULT_LOCKED.
	 */
	lock_page(page);

	/* we loop through the pagelist before adding in order
	to keep the pagelist sorted */
	list_for_each_entry(cur, &fbdefio->pagelist, lru) {
		/* this check is to catch the case where a new
		process could start writing to the same page
		through a new pte. this new access can cause the
		mkwrite even when the original ps's pte is marked
		writable */
		if (unlikely(cur == page))
			goto page_already_added;
		else if (cur->index > page->index)
			break;
	}

	list_add_tail(&page->lru, &cur->lru);

page_already_added:
	mutex_unlock(&fbdefio->lock);

	/* come back after delay to process the deferred IO */
	schedule_delayed_work(&info->deferred_work, fbdefio->delay);
	return VM_FAULT_LOCKED;
}

File: include/linux/workqueue.h

// schedule_delayed_work(&info->deferred_work, fbdefio->delay);
/**
 * schedule_delayed_work - put work task in global workqueue after delay
 * @dwork: job to be done
 * @delay: number of jiffies to wait or 0 for immediate execution
 *
 * After waiting for a given time this puts a job in the kernel-global
 * workqueue.
 */
static inline bool schedule_delayed_work(struct delayed_work *dwork,
					 unsigned long delay)
{
	return queue_delayed_work(system_wq, dwork, delay);
}

// queue_delayed_work(system_wq, &info->deferred_work, fbdefio->delay);
/**
 * queue_delayed_work - queue work on a workqueue after delay
 * @wq: workqueue to use
 * @dwork: delayable work to queue
 * @delay: number of jiffies to wait before queueing
 *
 * Equivalent to queue_delayed_work_on() but tries to use the local CPU.
 */
static inline bool queue_delayed_work(struct workqueue_struct *wq,
				      struct delayed_work *dwork,
				      unsigned long delay)
{
	return queue_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);
}

File: kernel/workqueue.c

// queue_delayed_work_on(WORK_CPU_UNBOUND, system_wq, &info->deferred_work, fbdefio->delay);
/**
 * queue_delayed_work_on - queue work on specific CPU after delay
 * @cpu: CPU number to execute work on
 * @wq: workqueue to use
 * @dwork: work to queue
 * @delay: number of jiffies to wait before queueing
 *
 * Returns %false if @work was already on a queue, %true otherwise.  If
 * @delay is zero and @dwork is idle, it will be scheduled for immediate
 * execution.
 */
bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
			   struct delayed_work *dwork, unsigned long delay)
{
	struct work_struct *work = &dwork->work;
	bool ret = false;
	unsigned long flags;

	/* read the comment in __queue_work() */
	local_irq_save(flags);

	if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
		__queue_delayed_work(cpu, wq, dwork, delay);
		ret = true;
	}

	local_irq_restore(flags);
	return ret;
}

// __queue_delayed_work(WORK_CPU_UNBOUND, system_wq, &info->deferred_work, fbdefio->delay);
static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
				struct delayed_work *dwork, unsigned long delay)
{
	struct timer_list *timer = &dwork->timer;
	struct work_struct *work = &dwork->work;

	WARN_ON_ONCE(timer->function != delayed_work_timer_fn ||
		     timer->data != (unsigned long)dwork);
	WARN_ON_ONCE(timer_pending(timer));
	WARN_ON_ONCE(!list_empty(&work->entry));

	/*
	 * If @delay is 0, queue @dwork->work immediately.  This is for
	 * both optimization and correctness.  The earliest @timer can
	 * expire is on the closest next tick and delayed_work users depend
	 * on that there's no such delay when @delay is 0.
	 */
	if (!delay) {
		__queue_work(cpu, wq, &dwork->work);
		return;
	}

	timer_stats_timer_set_start_info(&dwork->timer);

	dwork->wq = wq;
	dwork->cpu = cpu;
	timer->expires = jiffies + delay;

	if (unlikely(cpu != WORK_CPU_UNBOUND))
		add_timer_on(timer, cpu);
	else
		add_timer(timer);
}

File: kernel/timer.c

// add_timer(&info->deferred_work->timer);
/**
 * add_timer - start a timer
 * @timer: the timer to be added
 *
 * The kernel will do a ->function(->data) callback from the
 * timer interrupt at the ->expires point in the future. The
 * current time is 'jiffies'.
 *
 * The timer's ->expires, ->function (and if the handler uses it, ->data)
 * fields must be set prior calling this function.
 *
 * Timers with an ->expires field in the past will be executed in the next
 * timer tick.
 */
void add_timer(struct timer_list *timer)
{
	BUG_ON(timer_pending(timer));
	mod_timer(timer, timer->expires);
}

File: kernel/workqueue.c

void delayed_work_timer_fn(unsigned long __data)
{
        struct delayed_work *dwork = (struct delayed_work *)__data;

        /* should have been called from irqsafe timer with irq already off */
        __queue_work(dwork->cpu, dwork->wq, &dwork->work);
}

File: drivers/video/fb_defio.c

/* workqueue callback */
static void fb_deferred_io_work(struct work_struct *work)
{
	struct fb_info *info = container_of(work, struct fb_info,
						deferred_work.work);
	struct list_head *node, *next;
	struct page *cur;
	struct fb_deferred_io *fbdefio = info->fbdefio;

	/* here we mkclean the pages, then do all deferred IO */
	mutex_lock(&fbdefio->lock);
	list_for_each_entry(cur, &fbdefio->pagelist, lru) {
		lock_page(cur);
		page_mkclean(cur);
		unlock_page(cur);
	}

	/* driver's callback with pagelist */
	fbdefio->deferred_io(info, &fbdefio->pagelist);

	/* clear the list */
	list_for_each_safe(node, next, &fbdefio->pagelist) {
		list_del(node);
	}
	mutex_unlock(&fbdefio->lock);
}

File: fbtft-core.c

void fbtft_deferred_io(struct fb_info *info, struct list_head *pagelist)
{
	struct fbtft_par *par = info->par;
	struct page *page;
	unsigned long index;
	unsigned y_low=0, y_high=0;
	int count = 0;

	/* debug can be changed via sysfs */
	fbtft_debug_sync_value(par);

	/* Mark display lines as dirty */
	list_for_each_entry(page, pagelist, lru) {
		count++;
		index = page->index << PAGE_SHIFT;
		y_low = index / info->fix.line_length;
		y_high = (index + PAGE_SIZE - 1) / info->fix.line_length;
		fbtft_fbtft_dev_dbg(DEBUG_DEFERRED_IO, par, info->device, "page->index=%lu y_low=%d y_high=%d\n", page->index, y_low, y_high);
		if (y_high > info->var.yres - 1)
			y_high = info->var.yres - 1;
		if (y_low < par->dirty_lines_start)
			par->dirty_lines_start = y_low;
		if (y_high > par->dirty_lines_end)
			par->dirty_lines_end = y_high;
	}

	par->fbtftops.update_display(info->par);
}