Date: Sun, 26 Aug 2001 14:16:12 -0400 (EDT) From: Zhihui Zhang <zzhang@cs.binghamton.edu> To: freebsd-hackers@freebsd.org Subject: Re: ata0-master: non aligned DMA transfer attempted Message-ID: <Pine.SOL.4.21.0108261415301.9351-100000@opal>
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Thanks for your replay. I use gdb to find out that the buffer address is
not 16-byte aligned. This leads to a question as to how to align a
statically allocated data structure properly. Using union seems to be able
to align you on a long boundary (or even long long?), but that is not 16
byte aligned.
union {
my_data_structure_t xyz;
long pad;
}
The natural alignment seems to work only on primitive data types. If you
define:
unsigned char sector_buf[512];
It will not always be aligned on a 512 byte boundary, even 16-byte
alignment is not guaranteed. Is there a way to achieve this?
-Zhihui
On Fri, 24 Aug 2001, Julian Elischer wrote:
> Zhihui Zhang wrote:
> >
> > I write a program that writes into a raw device directly. Although the
> > program runs OK, the system prints messages like:
> >
> > ata0-master: non aligned DMA transfer attempted
> make sure your DMA buffer is alligned on a 64 byte boundary...
> (a page would be best)
> and that you are transferring an exact bultiple of 512 bytes.
>
> The DMA hardware on some macines cannot handle a buffer on less than 16 byte
> allignment, (some on odd allignment,.. (it's a bit hardware dependent).
>
> so be safe and allign your buffers.
>
>
> when it detects it cannot do it, i used PIO instead, so your data is still
> transferred...
>
> >
> > What exactly happens here? Is there any problem in my program?
> >
> > Thanks.
> >
> > -Zhihui
> >
> > To Unsubscribe: send mail to majordomo@FreeBSD.org
> > with "unsubscribe freebsd-hackers" in the body of the message
>
> --
> +------------------------------------+ ______ _ __
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> v
>
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