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SubjectRe: simple framebuffer slower by factor of 20, on socfpga (arm) platform
On 09/04/15 14:06, Pavel Machek wrote:
> On Tue 2015-04-07 14:19:33, Geert Uytterhoeven wrote:
>> Hi Pavel,
>>
>> On Tue, Apr 7, 2015 at 2:12 PM, Pavel Machek <pavel@ucw.cz> wrote:
>>> I have an socfpga board, which uses has simple framebuffer implemented
>>> in the FPGA. On 3.15, framebuffer is fast:
>>>
>>> root@wagabuibui:~# time cat /dev/fb0 > /dev/null
>>> real 0m 0.00s
>>> user 0m 0.00s
>>> sys 0m 0.00s
>>>
>>> on 3.18, this takes 220msec. Similar slowdown exists for
>>> writes. Simple framebuffer did not change at all between 3.15 and
>>> 3.18; resource flags of the framebuffer are still same (0x200).
>>>
>>> If I enable caching on 3.18, it speeds up a bit, to 70msec or
>>> so... Which means problem is not only in caching.
>>>
>>> Any ideas?
>>
>> My first guess was commit 67dc0d4758e5 ("vt_buffer: drop console buffer
>> copying optimisations"), but this was introduced only in v4.0-rc1.
>>
>> Just in case you encounter another performance regression after upgrading
>> to a more modern kernel ;-)
>
> :-). I did a git bisect, and it pointed to this. And reverting it
> indeed fixes the problem in 3.18. Problem is still there in 4.0.

Interesting. I can reproduce this also on TI's AM437x board, on 3.14
kernel. Without the patch:

# time cat /dev/fb0 > /dev/null
real 0m 0.01s
user 0m 0.00s
sys 0m 0.01s

With the patch:

# time cat /dev/fb0 > /dev/null
real 0m 0.19s
user 0m 0.01s
sys 0m 0.17s

> Archit do you know what is going on there? Should the revert be filled
> for 4.0?

(Cc'ing Archit's new email)

>
> Pavel
>
> commit 981409b25e2a99409b26daa67293ca1cfd5ea0a0
> Author: Archit Taneja <archit@ti.com>
> Date: Fri Nov 16 14:46:04 2012 +0530
>
> fbdev: arm has __raw I/O accessors, use them in fb.h
>
> This removes the sparse warnings on arm platforms:
>
> warning: cast removes address space of expression
>
> Signed-off-by: Archit Taneja <archit@ti.com>
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
> Cc: H Hartley Sweeten <hsweeten at visionengravers.com>
> Cc: Alexander Shiyan <shc_work@mail.ru>
> Cc: Russell King <linux@arm.linux.org.uk>


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