linux-kernel - [PATCH 3.16 062/328] udlfb: set optimal write delay

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Message-ID: <lsq.1544392233.608210304@decadent.org.uk>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Mikulas Patocka" <mpatocka@...hat.com>,
        "Bartlomiej Zolnierkiewicz" <b.zolnierkie@...sung.com>
Subject: [PATCH 3.16 062/328] udlfb: set optimal write delay

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mikulas Patocka <mpatocka@...hat.com>

commit bb24153a3f13dd0dbc1f8055ad97fe346d598f66 upstream.

The default delay 5 jiffies is too much when the kernel is compiled with
HZ=100 - it results in jumpy cursor in Xwindow.

In order to find out the optimal delay, I benchmarked the driver on
1280x720x30fps video. I found out that with HZ=1000, 10ms is acceptable,
but with HZ=250 or HZ=300, we need 4ms, so that the video is played
without any frame skips.

This patch changes the delay to this value.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/video/udlfb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/video/udlfb.h
+++ b/include/video/udlfb.h
@@ -88,7 +88,7 @@ struct dlfb_data {
 #define MIN_RAW_PIX_BYTES	2
 #define MIN_RAW_CMD_BYTES	(RAW_HEADER_BYTES + MIN_RAW_PIX_BYTES)
 
-#define DL_DEFIO_WRITE_DELAY    5 /* fb_deferred_io.delay in jiffies */
+#define DL_DEFIO_WRITE_DELAY    msecs_to_jiffies(HZ <= 300 ? 4 : 10) /* optimal value for 720p video */
 #define DL_DEFIO_WRITE_DISABLE  (HZ*60) /* "disable" with long delay */
 
 /* remove these once align.h patch is taken into kernel */