linux-kernel - Re: [PATCH] vt_buffer: drop console buffer copying optimisations

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Message-ID: <CA+55aFw75+RrQ7QfrTvsOAc0cKRuNASA9OUqwuwpA7n=QTTe5w@mail.gmail.com>
Date:	Thu, 29 Jan 2015 15:40:33 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Airlie <airlied@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.sf.net,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH] vt_buffer: drop console buffer copying optimisations

On Wed, Jan 28, 2015 at 8:11 PM, Dave Airlie <airlied@...hat.com> wrote:
>
> Linus, this came up a while back I finally got some confirmation
> that it fixes those servers.

I'm certainly ok with this. which way should it go in? The users are:

 - drivers/tty/vt/vt.c (Greg KH, "tty layer")

 - drivers/video/console/* (fbcon people: Tomi Valkeinen and friends)

and it might make sense to have *some* indication of how much worse
this makes fbcon performance in particular..

Greg/Tomi - the patch is removing this:

  #define scr_memcpyw(d, s, c) memcpy(d, s, c)
  #define scr_memmovew(d, s, c) memmove(d, s, c)
  #define VT_BUF_HAVE_MEMCPYW
  #define VT_BUF_HAVE_MEMMOVEW

from <linux/vt_buffer.h>, because some stupid graphics cards
apparently cannot handle 64-bit accesses of regular memcpy/memmove.

And on other setups, this will be the reverse: 8-bit accesses due to
using "rep movsb", which is the fast way to move/clear memory on
modern Intel CPU's, but is really wrong for MMIO where it will be slow
as hell.

So just getting rid of the memcpy/memmove is likely the right thing in
general, since the fallbacks go this the traditional 16-bit-at-a-time
way. And getting rid of the memcpy _may_ speed things up.

But if it slows things down, we might have to try something else. Like
saying "all cards we've ever seen have been ok with aligned 32-bit
accesses", and extend the open-coded scr_memcpy/memmove functions to
do that.

Hmm?

                           Linus
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