Re: [Xen-devel] [PATCH v4 2/5] lib: devres: add pcim_iomap_wc() variants

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

On Thu, Apr 30, 2015 at 11:26:47AM -0500, Bjorn Helgaas wrote:
> [+cc linux-pci]
> 
> Hi Luis,
> 
> On Wed, Apr 29, 2015 at 02:36:09PM -0700, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> > 
> > Now that we have pci_iomap_wc() add the respective devres helpers.
> 
> I guess I'm still confused about the relationship between pci_iomap_wc()
> and arch_phys_wc_add().
> 
> Do you expect every caller of pcim_iomap_wc() to also call
> arch_phys_wc_add()?

Yeap.

> If so, I'm not sure how pcim_iomap_wc() fits into the picture.  A driver
> can call both pcim_iomap_wc() and arch_phys_wc_add(), but the driver
> doesn't explicitly do the unmap, so where would the arch_phys_wc_del()
> happen?

As with other current drivers not using devres, upon exit or where they
would otherwise typically iounmap().

> If not, how does a driver know whether it should call arch_phys_wc_add()?

Sadly they'd have to figure it out, as Andy notes arch_phys_wc_add() is
a hack so I think we need to leave it as such and hope to see arch_phys_wc_add()
use phased as it won't be needed anymore really. arch_phys_wc_add() really 
should
only be used by device drivers that know that are working with non-PAT systems.
The code already takes care of this but since its an x86 write-combining hack
we should not consider meshing it with devres.

> > ...
> >  /**
> > + * pcim_iomap_wc_regions - Request and iomap PCI BARs with write-combining
> > + * @pdev: PCI device to map IO resources for
> > + * @mask: Mask of BARs to request and iomap
> > + * @name: Name used when requesting regions
> > + *
> > + * Request and iomap regions specified by @mask with a preference for
> > + * write-combining.
> > + */
> > +int pcim_iomap_wc_regions(struct pci_dev *pdev, int mask, const char *name)
> > +{
> > +   void __iomem * const *iomap;
> > +   int i, rc;
> > +
> > +   iomap = pcim_iomap_table(pdev);
> > +   if (!iomap)
> > +           return -ENOMEM;
> > +
> > +   for (i = 0; i < DEVICE_COUNT_RESOURCE; i++) {
> > +           unsigned long len;
> > +
> > +           if (!(mask & (1 << i)))
> > +                   continue;
> > +
> > +           rc = -EINVAL;
> > +           len = pci_resource_len(pdev, i);
> > +           if (!len)
> > +                   goto err_inval;
> > +
> > +           rc = pci_request_region(pdev, i, name);
> > +           if (rc)
> > +                   goto err_inval;
> > +
> > +           rc = -ENOMEM;
> > +           if (!pcim_iomap_wc(pdev, i, 0))
> > +                   goto err_region;
> 
> Is there a user for this?  Are there really devices where *all* the BARs
> can be mapped with WC?  Are there enough of them to make it worth adding
> this?

Not right now, I did this more to help with a friend who is testing one
driver for a feature. The driver is upstream but a way to make the feature
take effect only under certain conditions still would need to be done.

> I don't see users of either pcim_iomap_wc() or pcim_iomap_wc_regions() so
> far.  Did I miss them, or do you just expect them in the near future?

The later, and also I hate seeing folks later add code under EXPORT_SYMBOL()
rather than EXPORT_SYMBOL_GPL() so I figure I'd rather do it first. It happened
recently in my v1 series, someone beat me to a write-combining export symbol
and changed it to EXPORT_SYMBOL(). Feel free to drop this though but I hope
no one out there then tries to just add an EXPORT_SYMBOL() later for this...

 Luis

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