linux-kernel - Re: [patch] x86, voyager: fix ioremap

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Date:	Mon, 28 Apr 2008 07:10:06 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] x86, voyager: fix ioremap_nocache()

On Sun, 27 Apr 2008 18:39:24 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> James Bottomley wrote:
> > Here's another piece of the x86 API that's designed to be cached.
> > The dma_declare_coherent_memory() usually represents behind bridge
> > memory that's fully participatory in the coherence model.
> > 
> > Making it uncached damages the utility of this memory because doing
> > cacheline sized burst cycles when needed to it is far faster than
> > individual byte/word/quad writes.
> > 
> > Signed-off-by: James Bottomley
> > <James.Bottomley@...senPartnership.com>
> > 
> > ---
> > 
> > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> > index 388b113..df83ffd 100644
> > --- a/arch/x86/kernel/pci-dma.c
> > +++ b/arch/x86/kernel/pci-dma.c
> > @@ -214,7 +214,7 @@ int dma_declare_coherent_memory(struct device
> > *dev, dma_addr_t bus_addr, 
> >  	/* FIXME: this routine just ignores
> > DMA_MEMORY_INCLUDES_CHILDREN */ 
> > -	mem_base = ioremap(bus_addr, size);
> > +	mem_base = ioremap_cache(bus_addr, size);
> >  	if (!mem_base)
> >  		goto out;

this patch patch is likely broken on x86; or rather, anyone who uses it is...
thinking you can find cache coherent memory on a PCI or similar bus that is actually
cachable... keep dreaming. (for now; there's talk about extending PCI)


> I would rather change drivers to use ioremap_nocache(), and leave the 
> API as-is.
> 
> Isn't there Yet More Breakage in lib/iomap.c, given these new
> semantics?
> 
>          if (flags & IORESOURCE_MEM) {
>                  if (flags & IORESOURCE_CACHEABLE)
>                          return ioremap(start, len);
>                  return ioremap_nocache(start, len);
>          }
> 
> Any driver using pci_iomap() (libata, and others) is affected.

only if you map ROM's. Anything but ROMs you cannot set IORESOURCE_CACHEABLE
on... since PCI MMIO memory isn't cache coherent per se. (it's cache coherent on
x86 by virtue of being uncachable ;-)
Thankfully Linux doesn't do that.

> I disagree with this semantics change.  A number of code places _and 
> drivers_ GET IT RIGHT, and these are all broken now?

Can you list one that gets it actually right ?
(cachable pretty much means: "the cpu is the only one changing it AND there is no side effect of 
reading or writing")

-- 
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