Re: [PATCH 4/8] drm/fb-helper: Add fb_deferred_io support

From: Noralf TrÃnnes
Date: Fri Apr 22 2016 - 13:28:30 EST

Den 22.04.2016 19:05, skrev Daniel Vetter:
On Fri, Apr 22, 2016 at 04:17:14PM +0200, Noralf Trønnes wrote:
Den 22.04.2016 10:27, skrev Daniel Vetter:
On Thu, Apr 21, 2016 at 08:54:45PM +0200, Noralf Trønnes wrote:
Den 20.04.2016 17:25, skrev Noralf Trønnes:
This adds deferred io support if CONFIG_FB_DEFERRED_IO is enabled.
Accumulated fbdev framebuffer changes are signaled using the callback
(struct drm_framebuffer_funcs *)->dirty()

The drm_fb_helper_sys_*() functions will accumulate changes and
schedule fb_info.deferred_work _if_ fb_info.fbdefio is set.
This worker is used by the deferred io mmap code to signal that it
has been collecting page faults. The page faults and/or other changes
are then merged into a drm_clip_rect and passed to the framebuffer
dirty() function.

The driver is responsible for setting up the fb_info.fbdefio structure
and calling fb_deferred_io_init() using the provided callback:
(struct fb_deferred_io).deferred_io = drm_fb_helper_deferred_io;

Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
drivers/gpu/drm/drm_fb_helper.c | 119 +++++++++++++++++++++++++++++++++++++++-
include/drm/drm_fb_helper.h | 15 +++++
2 files changed, 133 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c

+ * drm_fb_helper_deferred_io() - (struct fb_deferred_io *)->deferred_io callback
+ * function
+ *
+ * This function always runs in process context (struct delayed_work) and calls
+ * the (struct drm_framebuffer_funcs)->dirty function with the collected
+ * damage. There's no need to worry about the possibility that the fb_sys_*()
+ * functions could be running in atomic context. They only schedule the
+ * delayed worker which then calls this deferred_io callback.
+ */
+void drm_fb_helper_deferred_io(struct fb_info *info,
+ struct list_head *pagelist)
+ struct drm_fb_helper *helper = info->par;
+ unsigned long start, end, min, max;
+ struct drm_clip_rect clip;
+ unsigned long flags;
+ struct page *page;
+ if (!helper->fb->funcs->dirty)
+ return;
+ spin_lock_irqsave(&helper->dirty_lock, flags);
+ clip = helper->dirty_clip;
+ drm_clip_rect_reset(&helper->dirty_clip);
+ spin_unlock_irqrestore(&helper->dirty_lock, flags);
+ min = ULONG_MAX;
+ max = 0;
+ list_for_each_entry(page, pagelist, lru) {
+ start = page->index << PAGE_SHIFT;
+ end = start + PAGE_SIZE - 1;
+ min = min(min, start);
+ max = max(max, end);
+ }
+ if (min < max) {
+ struct drm_clip_rect mmap_clip;
+ mmap_clip.x1 = 0;
+ mmap_clip.x2 = info->var.xres;
+ mmap_clip.y1 = min / info->fix.line_length;
+ mmap_clip.y2 = min_t(u32, max / info->fix.line_length,
+ info->var.yres);
+ drm_clip_rect_merge(&clip, &mmap_clip, 1, 0, 0, 0);
+ }
+ if (!drm_clip_rect_is_empty(&clip))
+ helper->fb->funcs->dirty(helper->fb, NULL, 0, 0, &clip, 1);
There is one thing I have wondered about when it comes to deferred io and
long run times for the defio worker with my displays:

Userspace writes to some pages then the deferred io worker kicks off and
runs for 100ms holding the page list mutex. While this is happening,
userspace writes to a new page triggering a page_mkwrite. Now this
function has to wait for the mutex to be released.

Who is actually waiting here and is there a problem that this can last
for 100ms?
No idea at all - I haven't looked that closely at fbdev defio. But one
reason we have an explicit ioctl in drm to flush out frontbuffer rendering
is exactly that flushing could take some time, and should only be done
once userspace has completed some rendering. Not right in the middle of an

I guess fix up your userspace to use dumb drm fb + drm dirtyfb ioctl?
Otherwise you'll get to improve fbdev defio, and fbdev is deprecated
subsystem and a real horror show. I highly recommend against touching it
I have tried to track the call chain and it seems to be part of the
page fault handler. Which means it's userspace wanting to write to the
page that has to wait. And it has to wait at some random point in
whatever rendering it's doing.

Unless someone has any objections, I will make a change and add a worker
like qxl does. This decouples the deferred_io worker holding the mutex
from the framebuffer flushing job. However I intend to differ from qxl in
that I will use a delayed worker (run immediately from the mmap side which
has already been deferred). Since I don't see any point in flushing the
framebuffer immediately when fbcon has put out only one glyph, might as
well defer it a couple of jiffies to be able to capture some more glyphs.

Adding a worker also means that udl doesn't have to initialize deferred_io
because we won't be using the deferred_work worker for flushing fb_*().
I'm confused ... I thought we already have enough workers? One in the
fbdev deferred_io implementation used by default. The other in case we get
a draw call from an atomic context.

This patch extend the use of the fbdev deferred_io worker to also handle
the fbdev drawing operations, which can happen in atomic context.
The qxl driver adds an extra worker (struct qxl_device).fb_work which is
used to flush the framebuffer. Both the mmap deferred_io (qxl_deferred_io())
code which is run by the deferred io worker and the fbdev drawing operations
(qxl_fb_fillrect() etc.) schedule this fb_work worker.

So this patch uses 1 worker, qxl uses 2 workers.

It's no big deal for me, fbtft has used 1 worker since 2013 without anyone
pointing out that this has been a problem. And and extra worker can be
added later without changing the drivers.
But since qxl used an extra worker I thought maybe there's a reason for
that and it would remove my worry about those page faults being held up.


Why do we need even more workers? Have you measured that you actually hit
this delay, or just conjecture from reading the code? Because my reading
says that the defio mmap support in fbdev already does what you want, and
should sufficiently coalesce mmap access. There's a delayed work/timer in
there to make sure it doesn't flush on the very first faulted page.

And yes, using drm from userspace is "The solution" here :-), however
I want to make the best out of fbdev since some of the tinydrm users
coming from drivers/staging/fbtft will probably continue with fbdev.


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