-----Original Message-----
From: Michael Kelley <redacted>

From: Wei Hu <redacted>  Sent: Tuesday, August 27, 2019 4:25 AM

quoted

Without deferred IO support, hyperv_fb driver informs the host to refresh
the entire guest frame buffer at fixed rate, e.g. at 20Hz, no matter there
is screen update or not. This patch supports deferred IO for screens in
graphics mode and also enables the frame buffer on-demand refresh. The
highest refresh rate is still set at 20Hz.

Currently Hyper-V only takes a physical address from guest as the starting
address of frame buffer. This implies the guest must allocate contiguous
physical memory for frame buffer. In addition, Hyper-V Gen 2 VMs only
accept address from MMIO region as frame buffer address. Due to these
limitations on Hyper-V host, we keep a shadow copy of frame buffer
in the guest. This means one more copy of the dirty rectangle inside
guest when doing the on-demand refresh. This can be optimized in the
future with help from host. For now the host performance gain from deferred
IO outweighs the shadow copy impact in the guest.

v2: Incorporated review comments from Michael Kelley
- Increased dirty rectangle by one row in deferred IO case when sending
to Hyper-V.
- Corrected the dirty rectangle size in the text mode.
- Added more comments.
- Other minor code cleanups.

Version history should go after the "---" below so it is not included in
the commit message.

[Wei Hu] 
I saw version history in the commit logs. 

quoted

Signed-off-by: Wei Hu <redacted>
---
 drivers/video/fbdev/Kconfig     |   1 +
 drivers/video/fbdev/hyperv_fb.c | 221 +++++++++++++++++++++++++++++---
 2 files changed, 202 insertions(+), 20 deletions(-)

+/* Deferred IO callback */
+static void synthvid_deferred_io(struct fb_info *p,
+				 struct list_head *pagelist)
+{
+	struct hvfb_par *par = p->par;
+	struct page *page;
+	unsigned long start, end;
+	int y1, y2, miny, maxy;
+	unsigned long flags;
+
+	miny = INT_MAX;
+	maxy = 0;
+
+	/*
+	 * Merge dirty pages. It is possible that last page cross
+	 * over the end of frame buffer row yres. This is taken care of
+	 * in synthvid_update function by clamping the y2
+	 * value to yres.
+	 */
+	list_for_each_entry(page, pagelist, lru) {
+		start = page->index << PAGE_SHIFT;
+		end = start + PAGE_SIZE - 1;
+		y1 = start / p->fix.line_length;
+		y2 = end / p->fix.line_length;
+		if (y2 > p->var.yres)
+			y2 = p->var.yres;

The above test seems contradictory to the comment that
says the clamping is done in synthvid_update().  Also, since
the above calculation of y2 is "inclusive", the clamping should
be done to yres - 1 in order to continue to be inclusive.  Then
when maxy + 1 is passed to synthvid_update() everything works
out correctly.

[Wei Hu] 
Actually the original code I sent out just works correctly.  It always get
the inclusive rectangle in the above loop, and only send one more extra
line (if y2 == yres) to sythvid_update() and it is clamped inside that 
function. Changing it to yres -1 and sending maxy + 1 to sytnvid_update()
makes it the same as the original code in this case, and would end up 
always copy and refresh one extra row when y2 < yres.

The comment I added was according to your last review comment asking
to add some comments explaining it. Maybe I mis-understood. I thought
since you wanted me to change to maxy + 1, the code could reach yres + 1
so it will be clamped in synthvid_update() to yres.

 Wei