Date: Wed, 29 Nov 2000 18:42:20 +0100
From: Vojtech Pavlik <vojtech@suse.cz>
To: Santiago Garcia Mantinan <manty@i.am>
Cc: linux-kernel@vger.kernel.org
Subject: Re: why volatile on vgacon.c?
Message-ID: <20001129184220.A204@suse.cz>
In-Reply-To: <20001129172415.A2171@mantianito.dyndns.org>
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In-Reply-To: <20001129172415.A2171@mantianito.dyndns.org>; from manty@i.am on Wed, Nov 29, 2000 at 05:24:15PM +0100
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On Wed, Nov 29, 2000 at 05:24:15PM +0100, Santiago Garcia Mantinan wrote:
> Hi!
> 
> I used to be able to run my 12 ethernet ports pentium based bridge without
> vga card, but with tty1, tty2, ... still working, as the kernel used to
> recognice a kind of a cga card on my machine even though there was none. But
> the kernel could write to the memory were the card was supposed to be, and
> so it worked.
> 
> That was on 2.2 series, but since I moved it to 2.4 series I don't have that
> cga card found anymore. I have looked on the kernel code and followed it to
> the __init function in vgacon.c, more concretely this piece of code...
> 
>         scr_writew(0xAA55, p);
>         scr_writew(0x55AA, p + 1);
>         if (scr_readw(p) != 0xAA55 || scr_readw(p + 1) != 0x55AA) {
> 
> Well, the thing is that this code and the code in this function is almost
> the same in 2.4 as in 2.2, however reading returns the written values on 2.2
> and different ones (0xffff) on 2.4
> 
> This is caused by the volatile declaration of *p on 2.4, so the questions
> are:
> 
> was the old (I have found a CGA) behaviour considered a bug and is the
> volatile declaration its fix?

Yes. The compiler was optimizing too much.

> If so, is there any way to have /dev/tty1 on a no graphic card i386 machine?
> (besides unvolatilizating *p wich works for me)

I think you can use serial console instead, if you have serial ports in
the machine.

-- 
Vojtech Pavlik
SuSE Labs
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