Subject: [PATCH 0/2] ioremap_wc on arm64

From: [email-masked] (Jayachandran C)
Date: Mon, 22 May 2017 14:21:00 +0000

On Mon, May 22, 2017 at 01:22:37PM +0100, Will Deacon wrote:
> On Mon, May 22, 2017 at 11:53:50AM +0000, Jayachandran C wrote:
> > On Mon, May 22, 2017 at 09:56:16AM +0100, Catalin Marinas wrote:
> > > On Mon, May 22, 2017 at 07:01:45AM +0000, Jayachandran C wrote:
> > > > From its definition, the device "gather" attribute seems to be a better
> > > > fit for implementing write combining mapping in ioremap_wc().  And on
> > > > ThunderX2, Device GRE mapping has optimizations that makes it much faster
> > > > than normal uncached mapping.
> > > > 
> > > > I am not sure of the reasoning behind the original decision to make
> > > > ioremap_wc use "Normal Non-Cached" attribute, since all the other variants
> > > > of ioremap use device attributes, and ioremap_wc looks like an exception.
> > > 
> > > The reason we kept it as Normal NC is that Device_GRE does not allow
> > > unaligned accesses.
> > 
> > There does not to be an expectation to have unaligned access on __iomem
> > pointers. I also see that memremap can call ioremap_wc (which is a Normal
> > mapping) or ioremap_wt(which is a Device mapping), so that is inconsistent
> > as well.
> > 
> > Was this added for a specific use case? Also, do you think this patchset
> > is acceptable?
> 
> I think it was used for framebuffers. Note that using normal-NC also aligns
> with arch/arm/ and, since normal-NC is strictly a more relaxed memory type
> than Device-GRE, I'm really not keen making on this change.

If there is a hardware requirement that Normal-NC has to be implemented
as more relaxed attribute than Device GRE, I can go back to the hardware
team on this. I did not see any text in the ARM ARM requiring this.

Even if that is the case, having Normal attribute for ioremap just for
ioremap_wc is inconsistent. The Device Gathering attribute by its definition
is exactly what write combining requires. The memremap() API looks like a
better way to expose Normal-NC mapping (with additional features like
speculation etc.) if implemented correctly.

Also, patch 1/2 should be useful anyway - can that be picked up?

Thanks,
JC.