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

[Arnd Bergmann <arnd@arndb.de>]

On Thursday 09 April 2015 14:34:26 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@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.
> 
> 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...

Could memcpy() cause alignment traps here if the fb pointer is unaligned
and uncached?

	Arnd