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

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Date:	Thu, 3 May 2012 11:46:32 -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 06:21:27PM +0000, Bedia, Vaibhav wrote:
> On Thu, May 03, 2012 at 21:39:18, Mark A. Greer wrote:
> > 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.
> > 
> 
> So, if I understood this correctly, it's effectively like blocking a low power
> state transition (here wfi execution) when EMAC is active?

Assuming "it" is my patch, correct.

Mark
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