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

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:

quoted

From: "Luis R. Rodriguez" <redacted>

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.

quoted

...
 /**
+ * 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