linux/drivers/video
Dave Airlie dd0c41f8a7 efifb: allow user to disable write combined mapping.
This patch allows the user to disable write combined mapping
of the efifb framebuffer console using an nowc option.

A customer noticed major slowdowns while logging to the console
with write combining enabled, on other tasks running on the same
CPU. (10x or greater slow down on all other cores on the same CPU
as is doing the logging).

I reproduced this on a machine with dual CPUs.
Intel(R) Xeon(R) CPU E5-2609 v3 @ 1.90GHz (6 core)

I wrote a test that just mmaps the pci bar and writes to it in
a loop, while this was running in the background one a single
core with (taskset -c 1), building a kernel up to init/version.o
(taskset -c 8) went from 13s to 133s or so. I've yet to explain
why this occurs or what is going wrong I haven't managed to find
a perf command that in any way gives insight into this.

    11,885,070,715      instructions              #    1.39  insns per cycle
vs
    12,082,592,342      instructions              #    0.13  insns per cycle

is the only thing I've spotted of interest, I've tried at least:
dTLB-stores,dTLB-store-misses,L1-dcache-stores,LLC-store,LLC-store-misses,LLC-load-misses,LLC-loads,\mem-loads,mem-stores,iTLB-loads,iTLB-load-misses,cache-references,cache-misses

For now it seems at least a good idea to allow a user to disable write
combining if they see this until we can figure it out.

Note also most users get a real framebuffer driver loaded when kms
kicks in, it just happens on these machines the kernel didn't support
the gpu specific driver.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Peter Jones <pjones@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
2017-07-31 18:45:41 +02:00
..
backlight video: adp8870: move header file out of I2C realm 2017-07-06 08:58:39 +01:00
console mdacon: replace MDA_ADDR macro by inline function 2017-06-14 17:40:56 +02:00
fbdev efifb: allow user to disable write combined mapping. 2017-07-31 18:45:41 +02:00
logo video/logo: tidyup fb_logo_late_init initcall timing 2017-03-20 18:31:00 +01:00
display_timing.c
hdmi.c video: Add new aspect ratios for HDMI 2.0 2016-10-17 14:23:46 +02:00
Kconfig fbdev: sh_mipi_dsi: remove driver 2016-05-10 11:53:38 +03:00
Makefile fbdev: sti: don't select CONFIG_VT 2017-05-05 17:25:12 +02:00
of_display_timing.c Merge branch 'drm/next/du' of git://linuxtv.org/pinchartl/media into drm-next 2016-11-16 09:39:21 +10:00
of_videomode.c video: Fix possible leak in of_get_videomode() 2015-08-10 15:11:12 +03:00
vgastate.c fbdev: vgastate: remove trailing whitespaces 2015-01-13 12:53:25 +02:00
videomode.c