Re: [Xen-devel] [PATCH v4 2/2] IB/qib: use arch_phys_wc_add()

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Wed, Apr 22, 2015 at 12:57:18PM -0400, Doug Ledford wrote:
> On Wed, 2015-04-22 at 17:33 +0200, Luis R. Rodriguez wrote:
> > On Wed, Apr 22, 2015 at 09:54:38AM -0400, Doug Ledford wrote:
> > > On Tue, 2015-04-21 at 14:50 -0700, Luis R. Rodriguez wrote:
> > > 
> > > This:
> > > > +       /* MTRR was used if this is non-zero */
> > > > +       if (!dd->wc_cookie)
> > > >                 vma->vm_page_prot = 
> > > > pgprot_writecombine(vma->vm_page_prot);
> > > 
> > > And this:
> > > > +               dd->wc_cookie = arch_phys_wc_add(pioaddr, piolen);
> > > > +               if (dd->wc_cookie < 0)
> > > > +                       ret = -EINVAL;
> > > 
> > > don't agree on what wc_cookie will be on error.
> > 
> > Can you elaborate? The one below is the one that starts things,
> > and arch_phys_wc_add() will return 0 on PAT systems. For non-PAT
> > systems it will return a number > 0 *iff* a valid MTRR was added.
> > It will return negative onloy on error then.
> > 
> > The change above is meant to replace a check put in place to see
> > if PAT was enabled. The way we replace this is to ensure that
> > arch_phys_wc_add() returned 0.
> > 
> > If you disagree it'd be great if you can elaborate why.
> 
> Maybe I'm missing something, but in qib_enable_wc() you store the return
> from arch_phys_wc_add into wc_cookie.  That return is negative,

If and only if the system was non-PAT and mtrr_add() failed.

>  so you
> return from qib_enable_wc() to qib_init_one(), they see the ret value,
> they print out a warning about bad performance, then they clear the
> return value and continue with device initialization.
> 
> In all of this though, wc_cookie is never cleared and so it still has
> the error condition in it.  Then, much later at run time, you call
> mmap_piobufs() and you check the contents of wc_cookie, and if it's
> non-0 (which is still will be), you do the wrong thing, right?

Originally the code had it to run pgprot_writecombine() if PAT was going to be
used. After the code changes we check for !cookie which will be true when
cookie is 0 only. In case the cookie was an error, that is if mtrr_add()
failed, then this code would not run because (!negative) is false. The goal was
to trigger a run if the cookie was 0, which can only happen if PAT was enabled.

Please let me know, I'd like to get this right too.

> And what
> about at shutdown when you call qib_disable_wc() and your cookie still
> has an error code in it as well?

Well fortunately arch_phys_wc_del(negative) and arch_phys_wc_del(0) will be
a no-op. Its what helps us remove so much clutter.

 Luis

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