Re: simple framebuffer slower by factor of 20, on socfpga (arm) platform

From: Archit Taneja
Date: Fri Apr 10 2015 - 03:06:16 EST

On 04/09/2015 05:04 PM, Tomi Valkeinen wrote:
On 09/04/15 14:21, Tomi Valkeinen wrote:
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@xxxxxx> 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.

The difference is probably caused by memcpy() vs memcpy_fromio(). The
comment above memcpy_fromio() says "This needs to be optimized". I think
generally speaking memcpy_fromio() is correct for a framebuffer.

That said, if the fb is in RAM, and is only written by the CPU, I think
a normal memcpy() for fb_memcpy_fromfb() should be fine...

I didn't test for performance regressions when I posted this patch.

A look at _memcpy_fromio in arch/arm/kernel/io.c shows that readb() is used all the time, even when the source and destination addresses are aligned for larger reads to be possible. Other archs seem to use readl() or readq() when they can. Maybe that makes memcpy_fromio slower than the implementation of memcpy on arm?


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