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

[Doug Ledford <dledford@xxxxxxxxxx>]

On Wed, 2015-04-22 at 19:37 +0200, Luis R. Rodriguez wrote:
> 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.

OK, the logic works, but as much as anything, it's the comment that's
misleading.  The code would be clearer with a comment like this:

/* We used PAT if wc_cookie == 0 */
if (!dd->wc_cookie) {

That would be more accurate as well since the original comment didn't
account for the possible error code in wc_cookie, so it's possible you
didn't use either PAT or wc if you have that error code.

> 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.

OK.

-- 
Doug Ledford <dledford@xxxxxxxxxx>
              GPG KeyID: 0E572FDD

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