From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <dd6fe68a0812051027t5661d7ebs81cce9a4ca0a6b7f@mail.gmail.com>
Date: Fri,  5 Dec 2008 10:27:14 -0800
From: "Russ Cox" <rsc@swtch.com>
To: "Fans of the OS Plan 9 from Bell Labs" <9fans@9fans.net>
Subject: Re: [9fans] image/memimage speed
In-Reply-To: <39cb2be32e592403f7336c6200cf56a3@quanstro.net>
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	 <39cb2be32e592403f7336c6200cf56a3@quanstro.net>
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> i don't think this is the case.  if you recall from the original
> post, i have used the pat registers to set up memory types on
> a pcie card and i do see dramatic speedups for drawing to
> the screen.  however, reading from the screen is just as slow
> as before.

I think the problem of

> can you explain what the downside of double-buffering
> would be?  it's not like the days where we asked, hey buddy,
> have you got 4 megs to spare?

you mean using a soft screen (a kernel copy of the video
memory, so that you only ever write to the video card).
double buffering is switching the screen between two
different copies of the screen image, only ever drawing
on the one that is not currently on the screen.

in answer to your question, that might be a fine thing to do
now that memory is more plentiful.  no one has been bothered
enough to do it.  you would lose the hardware acceleration
for fill and scroll, since you can't have the video card editing
the frame buffer directly--your copy would be out of sync.
on the other hand, writes are so fast that it probably wouldn't
matter.  the win for hw scroll is not reading from the frame buffer.

i think it's a pretty trivial change, since the relevant code
is all there for non-direct-mapped frame buffers anyway.

go for it.

russ