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

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Date:	Tue, 7 Apr 2015 14:19:33 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Marek Vasut <marek.vasut@...il.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Jingoo Han <jg1.han@...sung.com>,
	Rob Clark <robdclark@...il.com>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: simple framebuffer slower by factor of 20, on socfpga (arm) platform

Hi Pavel,

On Tue, Apr 7, 2015 at 2:12 PM, Pavel Machek <pavel@....cz> wrote:
> I have an socfpga board, which uses has simple framebuffer implemented
> in the FPGA. On 3.15, framebuffer is fast:
>
> root@...abuibui:~# 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 ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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