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Date: Fri, 15 May 2009 14:16:42 +0400
From: Stanislav Sedov <stas@FreeBSD.org>
To: "M. Warner Losh" <imp@bsdimp.com>
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Subject: Re: svn commit: r192027 - head/sys/arm/at91
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On Thu, 14 May 2009 23:35:36 -0600 (MDT)
"M. Warner Losh" <imp@bsdimp.com> mentioned:

> In message: <20090515092205.6f6d06fa.stas@FreeBSD.org>
>             Stanislav Sedov <stas@FreeBSD.org> writes:
> : On Thu, 14 May 2009 21:37:12 -0600 (MDT)
> : "M. Warner Losh" <imp@bsdimp.com> mentioned:
> : 
> : > In message: <200905122114.n4CLEag9033208@svn.freebsd.org>
> : >             Stanislav Sedov <stas@FreeBSD.org> writes:
> : > : @@ -926,6 +937,7 @@ atestart_locked(struct ifnet *ifp)
> : > :  		 * tell the hardware to xmit the packet.
> : > :  		 */
> : > :  		WR4(sc, ETH_TAR, segs[0].ds_addr);
> : > : +		BARRIER(sc, ETH_TAR, 8, BUS_SPACE_BARRIER_WRITE);
> : > :  		WR4(sc, ETH_TCR, segs[0].ds_len);
> : > 
> : > Why is a barrier needed here?
> : > 
> : Writing the TCR register triggers the transmit, so it had to be written
> : strongly after the TAR register. That's why I added the barrier here.
> 
> Then shouldn't the barrier be after TCR write?  Or does this ensure
> that the write is before TCR?
> 

Yeah, this barrier is to ensure that the TCR register gets written after the
TAR register has been written, not before. I don't think an additional barrier
is needed after the TCR write.

-- 
Stanislav Sedov
ST4096-RIPE

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