netdev - Re: [PATCH] net: davinci_emac: Add pre_open, post

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Date:	Thu, 3 May 2012 09:09:18 -0700
From:	"Mark A. Greer" <mgreer@...malcreek.com>
To:	"Bedia, Vaibhav" <vaibhav.bedia@...com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] net: davinci_emac: Add pre_open, post_stop platform
 callbacks

On Thu, May 03, 2012 at 10:44:44AM +0000, Bedia, Vaibhav wrote:
> On Thu, May 03, 2012 at 05:17:18, Mark A. Greer wrote:
> > From: "Mark A. Greer" <mgreer@...malcreek.com>
> > 
> > The davinci EMAC driver has been incorporated into the am35x
> > family of SoC's which is OMAP-based.  The incorporation is
> > incomplete in that the EMAC cannot unblock the [ARM] core if
> > its blocked on a 'wfi' instruction.  This is an issue with
> > the cpu_idle code because it has the core execute a 'wfi'
> > instruction.
> > 
> > To work around this issue, add platform data callbacks which
> > are called at the beginning of the open routine and at the
> > end of the stop routine of the davinci_emac driver.  The
> > callbacks allow the platform code to issue disable_hlt() and
> > enable_hlt() calls appropriately.  Calling disable_hlt()
> > prevents cpu_idle from issuing the 'wfi' instruction.
> > 
> > It is not sufficient to simply call disable_hlt() when
> > there is an EMAC present because it could be present but
> > not actually used in which case, we do want the 'wfi' to
> > be executed.
> > 
> 
> Are you trying to say that if ARM executes _just_ wfi and _absolutely
> nothing else_ is done in the OMAP PM code, EMAC stops working?

No, I'm saying the EMAC can't wake the core from the wfi so if nothing
else happens in the system, its effectively hung.  If something else
does happen in the system (e.g., a timer expires), the the system is
extremely slow because because its only waking up when a timer (or
something else wakes it up--but not net traffic).  This is very apparent
when using an nfs-mounted rootfs. It doesn't hang but its extremely
slow because occasionally something else wakes up the core but it
spends most of its time stuck in the wfi when it should be handling
net/nfs traffic.

> However, if this is indeed the case, then probably a better solution would be
> to invoke disable_hlt() from the board file when EMAC support is compiled in.

Kevin addressed this one.  Thanks Kevin.

Mark
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