Re: [Xen-devel] [PATCH v4 1/5] pci: add pci_iomap_wc() variants

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

On Thu, Apr 30, 2015 at 10:59:17AM -0500, Bjorn Helgaas wrote:
> [+cc linux-pci]
> 
> Hi Luis,
> 
> On Wed, Apr 29, 2015 at 02:36:08PM -0700, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> > 
> > This allows drivers to take advantage of write-combining
> > when possible. Ideally we'd have pci_read_bases() just
> > peg an IORESOURCE_WC flag for us 
> 
> This makes it sound like pci_read_bases() could do a better job
> if we just tried harder, but I don't think that's the case.  All
> pci_read_bases() can do is look at the bits in the BAR.  For
> memory BARs, there's a "prefetchable" bit and a "64-bit" bit.
> 
> If you just want to complain that the PCI spec didn't define a
> way for software to discover whether a BAR can be mapped with WC,
> that's fine, but it's misleading to suggest that pci_read_bases()
> could figure out WC without some help from the spec.

You're right sorry about that, in my original patch this was more
of a question and I did not have a full answer for but mst had
clarified before the spec doesn't allow for this [0] and you are
confirming this now as well.

[0] https://lkml.org/lkml/2015/4/21/714

I'll update the patch and at least document we did think about
this and that its a shortcoming of the spec.

> > but where exactly
> > video devices memory lie varies *largely* and at times things
> > are mixed with MMIO registers, sometimes we can address
> > the changes in drivers, other times the change requires
> > intrusive changes.
> > 
> > Although there is also arch_phys_wc_add() that makes use of
> > architecture specific write-combining alternatives (MTRR on
> > x86 when a system does not have PAT) we void polluting
> > pci_iomap() space with it and force drivers and subsystems
> > that want to use it to be explicit.
> 
> I'm not quite sure I understand the point you're making here
> about not polluting pci_iomap_wc() with arch_phys_wc_add().  I
> think the choice is for a driver to do either this:
> 
>   info->screen_base = pci_iomap_wc(dev, 0, 0);
> 
> or this:
> 
>   info->screen_base = pci_iomap_wc(dev, 0, 0);
>   par->wc_cookie = arch_phys_wc_add(pci_resource_start(dev, 0),
>                                   pci_resource_len(dev, 0));
> 
> The driver is *already* being explicit because it calls
> pci_iomap_wc() instead of pci_iomap().
> 
> It seems like it would be ideal if ioremap_wc() could call
> arch_phys_wc_add() internally.

Indeed, that's what I was alluding to.

> Doesn't any caller of
> arch_phys_wc_add() have to also do some sort of ioremap()
> beforehand? 

This is not a requirement as the physical address is used,
not the virtual address.

> I assume there's some reason for separating them,

Well a full sweep to change to arch_phys_wc_add() was never done,
consider this part of the last effort to do so. In retrospect now
that I've covered all other drivers in 12 different series of patches
I think its perhaps best to not mesh them together as we're phasing
out MTRR and the only reason to have arch_phys_wc_add() is for MTRR
which is legacy.

I'll update the commit log to mention that.

> and I see that the current arch_phys_wc_add() requires the caller
> to store a handle, but doing both seems confusing.

That's just a cookie so that later when we undo the driver we can
tell the backend to remove it.

> > There are a few motivations for this:
> > 
> > a) Take advantage of PAT when available
> > 
> > b) Help bury MTRR code away, MTRR is architecture specific and on
> >    x86 its replaced by PAT
> > 
> > c) Help with the goal of eventually using _PAGE_CACHE_UC over
> >    _PAGE_CACHE_UC_MINUS on x86 on ioremap_nocache() (see commit
> >    de33c442e titled "x86 PAT: fix performance drop for glx,
> >    use UC minus for ioremap(), ioremap_nocache() and
> >    pci_mmap_page_range()")
> 
> I think these are now _PAGE_CACHE_MODE_UC and
> _PAGE_CACHE_MODE_UC_MINUS, right? 

Indeed, thanks, I'll fix that in the commit log.

> > ...
> 
> > +void __iomem *pci_iomap_wc_range(struct pci_dev *dev,
> > +                            int bar,
> > +                            unsigned long offset,
> > +                            unsigned long maxlen)
> > +{
> > +   resource_size_t start = pci_resource_start(dev, bar);
> > +   resource_size_t len = pci_resource_len(dev, bar);
> > +   unsigned long flags = pci_resource_flags(dev, bar);
> > +
> > +   if (len <= offset || !start)
> > +           return NULL;
> > +   len -= offset;
> > +   start += offset;
> > +   if (maxlen && len > maxlen)
> > +           len = maxlen;
> > +   if (flags & IORESOURCE_IO)
> > +           return __pci_ioport_map(dev, start, len);
> 
> Is there any point in checking for IORESOURCE_IO?  If a driver
> calls pci_iomap_wc_range(), I assume it already knows this is an
> IORESOURCE_MEM BAR, so if we see IORESOURCE_IO here we should
> just return an error, i.e., NULL.

Agreed, will fix with all the other changes on the commit log and
repost. I won't repost the full series but just this one patch as
a v5.

Thanks for the review.

 Luis

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