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

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Message-ID: <20150224164914.GA6683@amd>
Date:	Tue, 24 Feb 2015 17:49:15 +0100
From:	Pavel Machek <pavel@....cz>
To:	Dave Airlie <airlied@...hat.com>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.sf.net
Subject: Re: [PATCH] vt_buffer: drop console buffer copying optimisations

On Thu 2015-01-29 14:11:25, Dave Airlie wrote:
> These two copy to/from VGA memory, however on the Silicon
> Motion SMI750 VGA card on a 64-bit system cause console corruption.
> 
> This is due to the hw being buggy and not handling a 64-bit transaction
> correctly.
> 
> We could try and create a 32-bit version of these routines,
> but I'm not sure the optimisation is worth much today.
> 
> Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1132826
> 
> Tested-by: Huawei engineering.
> Signed-off-by: Dave Airlie <airlied@...hat.com>

Actually... are you sure this is right fix?

IOW can gcc do the optimalization behind your back and still break the
buggy card?
								Pavel

> diff --git a/include/linux/vt_buffer.h b/include/linux/vt_buffer.h
> index 057db7d..f38c10b 100644
> --- a/include/linux/vt_buffer.h
> +++ b/include/linux/vt_buffer.h
> @@ -21,10 +21,6 @@
>  #ifndef VT_BUF_HAVE_RW
>  #define scr_writew(val, addr) (*(addr) = (val))
>  #define scr_readw(addr) (*(addr))
> -#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
>  #endif
>  
>  #ifndef VT_BUF_HAVE_MEMSETW

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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