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efifb: allow user to disable write combined mapping.

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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>
This commit is contained in:
Dave Airlie 2017-07-31 18:45:41 +02:00 committed by Bartlomiej Zolnierkiewicz
parent 7b4dfbe788
commit dd0c41f8a7
2 changed files with 13 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/fb/efifb.txt
View File

@ -27,5 +27,11 @@ You have to add the following kernel parameters in your elilo.conf:
Macbook Pro 17", iMac 20" :
video=efifb:i20
Accepted options:
nowc Don't map the framebuffer write combined. This can be used
to workaround side-effects and slowdowns on other CPU cores
when large amounts of console data are written.
--
Edgar Hucek <gimli@dark-green.com>

8
drivers/video/fbdev/efifb.c
View File

@ -17,6 +17,7 @@
#include <asm/efi.h>
static bool request_mem_succeeded = false;
static bool nowc = false;
static struct fb_var_screeninfo efifb_defined = {
.activate = FB_ACTIVATE_NOW,
@ -99,6 +100,8 @@ static int efifb_setup(char *options)
screen_info.lfb_height = simple_strtoul(this_opt+7, NULL, 0);
else if (!strncmp(this_opt, "width:", 6))
screen_info.lfb_width = simple_strtoul(this_opt+6, NULL, 0);
else if (!strcmp(this_opt, "nowc"))
nowc = true;
}
}
@ -255,7 +258,10 @@ static int efifb_probe(struct platform_device *dev)
info->apertures->ranges[0].base = efifb_fix.smem_start;
info->apertures->ranges[0].size = size_remap;
info->screen_base = ioremap_wc(efifb_fix.smem_start, efifb_fix.smem_len);
if (nowc)
info->screen_base = ioremap(efifb_fix.smem_start, efifb_fix.smem_len);
else
info->screen_base = ioremap_wc(efifb_fix.smem_start, efifb_fix.smem_len);
if (!info->screen_base) {
pr_err("efifb: abort, cannot ioremap video memory 0x%x @ 0x%lx\n",
efifb_fix.smem_len, efifb_fix.smem_start);