lkml.org 
[lkml]   [2015]   [Jul]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v8 0/9] pci: add pci_iomap_wc() and pci_ioremap_wc_bar()
On Fri, Jun 26, 2015 at 12:12:06PM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2015-06-24 at 18:22 -0700, Luis R. Rodriguez wrote:
> > Although I had test compiled this before just to be safe I went ahead and
> > successfully test-compiled this set with allmodconfig, specially since I've now
> > removed the exports for the devres routines. Please let me know if these might
> > be able to go through you or if there are any questions. I will note the recent
> > discussion with Benjamin over the v7 series concluded that the ideas we both
> > were alluding to, on automating instead the WC effects for devices seems a bit
> > too idealistic for PCI / PCIE for now, but perhaps we should at least consider
> > this in the future for userspace mmap() calls [4].
>
> So I've been trying to figure out how to make this practically work for us (powerpc).
>
> writel() will never write combine for us, it uses too heavy barriers.
>
> writel_relaxed() today is identical to writel() but we can change it.
>
> The problem is that switching to G=0 mappings (which is what provides us with write
> combining) also architecturally enables prefetch and speculative loads... and again
> architecturally (the implementations may differ), kills the effect of the lightweight
> io barrier eieio which we would have to use in readl_relaxed() and writel_relaxed()
> to provide their normal semantics.
>
> So it boils down to: Can we modify the documentation of readl_relaxed() and writel_relaxed()
> to define them as being even further relaxed when using a "wc" mapping ?
>
> Otherwise, the only way out I see for us on powerpc is to bias massively writel_relaxed()
> against real_relaxed() by putting heavy barriers around the load in the latter so we can
> keep them completely out of the former and still enable wc.

Depends if you semantically then also are implicating its use for the ioremap_wc()
area and if we've ensured we've visited all other possibilities to avoid this. Instead
of replying here though it seems we have a large general ioremap() semantic discussion
ongoing on another thread which is far ahead of this one and more generalized. Mind
following up there, seems the party is there:

http://lkml.kernel.org/r/20150707160703.GR7021@wotan.suse.de

Luis


\
 
 \ /
  Last update: 2015-07-07 18:41    [W:0.254 / U:0.516 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site