RE: [PATCH v3] tpm: Enable CLKRUN protocol for Braswell systems

[Shaikh, Azhar]

> -----Original Message-----
> From: Alan Cox [mailto:gnomes@xxxxxxxxxxxxxxxxxxx]
> Sent: Thursday, June 8, 2017 11:23 AM
> To: Shaikh, Azhar <azhar.shaikh@xxxxxxxxx>
> Cc: jarkko.sakkinen@xxxxxxxxxxxxxxx; jgunthorpe@xxxxxxxxxxxxxxxxxxxx;
> tpmdd-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> security-module@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3] tpm: Enable CLKRUN protocol for Braswell systems
> 
> > > > +	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(). If it doesn't then you need to issue
> whatever access is needed to the fabric to ensure the post completed (eg for
> PCI if you do an MMIO write you must do an MMIO read from the same
> devfn).
> 
> Secondly outb(0x80, 0xCC) doesn't write 0xCC to port 0x80. It writes 0x80 to
> port 0xCC !
> 

Oops my bad! I got that reversed. Will change it.

> Alan