On Wed, Jan 28, 2015 at 8:11 PM, Dave Airlie <airlied@xxxxxxxxxx> 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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