linux-kernel - Re: [PATCH v3] tpm: Enable CLKRUN protocol for Braswell systems

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Date:   Sat, 10 Jun 2017 14:06:56 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "Shaikh, Azhar" <azhar.shaikh@...el.com>,
        "tpmdd-devel@...ts.sourceforge.net" 
        <tpmdd-devel@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v3] tpm: Enable CLKRUN protocol for Braswell systems

On Thu, Jun 08, 2017 at 12:39:20PM -0600, Jason Gunthorpe wrote:
> On Thu, Jun 08, 2017 at 07:22:59PM +0100, Alan Cox wrote:
> > > > > +	outb(0x80, 0xCC);
> > > > > +
> > > > > +	/* Make sure the above write is completed */
> > > > > +	wmb();  
> > > > 
> > > > Why the wmb(). It doesn't do what the comment says! Also this code is x86
> > > > specific
> > > > 
> > > >   
> > > 
> > > Memory barrier to enforce the order so that the outb() is
> > > completed, which ensures that the LPC clocks are running before
> > > sending any TPM command.
> >
> > wmb() doesn't do that. It merely ensures that the write has been posted
> > to the fabric. If as I suspect your LPC bus implements outb() as a
> > non-posted write you don't need the wmb().
> 
> I think the point here is to bootstrap the sleeping LPC bus clock
> before a TPM command is issued, presumably because the auto-wakeup circuit
> is busted or something.
> 
> For that purpose all that should be required is strong ordering of the
> outb relative to the other TPM commands at the LPC interface FIFO. I
> also think the wmb is not needed because outb is already defined to be
> strongly in order with respect to writel/readl ?
> 
> Jason

writel AFAIK guarantees by itself strong RW ordering.

/Jarkko