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

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Message-ID: <20150129235742.GB14741@kroah.com>
Date:	Thu, 29 Jan 2015 15:57:42 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Airlie <airlied@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.sf.net, Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH] vt_buffer: drop console buffer copying optimisations

On Thu, Jan 29, 2015 at 03:40:33PM -0800, Linus Torvalds wrote:
> 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?

I can take this through the tty tree, but can I put it in linux-next and
wait for the 3.20 merge window to give people who might notice a
slow-down a chance to object?

thanks,

greg k-h
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