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

[Bjorn Helgaas <bhelgaas@xxxxxxxxxx>]

[+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.

> 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.  Doesn't any caller of
arch_phys_wc_add() have to also do some sort of ioremap()
beforehand?  I assume there's some reason for separating them,
and I see that the current arch_phys_wc_add() requires the caller
to store a handle, but doing both seems confusing.

> 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? 

> ...

> +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.

> +     if (flags & IORESOURCE_MEM)
> +             return ioremap_wc(start, len);
> +     /* What? */
> +     return NULL;
> +}
> +EXPORT_SYMBOL_GPL(pci_iomap_wc_range);

Bjorn

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