linux-kernel - Re: [PATCH v4 2/5] lib: devres: add pcim_iomap

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Message-ID: <20150430172723.GU5622@wotan.suse.de>
Date:	Thu, 30 Apr 2015 19:27:23 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>, mst@...hat.com,
	plagnioj@...osoft.com, tomi.valkeinen@...com, airlied@...ux.ie,
	daniel.vetter@...el.com, linux-fbdev@...r.kernel.org,
	luto@...capital.net, cocci@...teme.lip6.fr,
	linux-kernel@...r.kernel.org, Toshi Kani <toshi.kani@...com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juergen Gross <jgross@...e.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...hat.com>,
	Antonino Daplas <adaplas@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>, venkatesh.pallipadi@...el.com,
	Stefan Bader <stefan.bader@...onical.com>,
	Ville Syrjälä <syrjala@....fi>,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Borislav Petkov <bp@...e.de>, Davidlohr Bueso <dbueso@...e.de>,
	konrad.wilk@...cle.com, ville.syrjala@...ux.intel.com,
	david.vrabel@...rix.com, jbeulich@...e.com,
	Roger Pau Monné <roger.pau@...rix.com>,
	xen-devel@...ts.xensource.com, linux-pci@...r.kernel.org
Subject: 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:
> > From: "Luis R. Rodriguez" <mcgrof@...e.com>
> > 
> > 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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