[PATCH 0/2] ioremap_wc on arm64

[Catalin Marinas]

On Tue, May 23, 2017 at 10:09:03AM +0000, Jayachandran C wrote:
> On Mon, May 22, 2017 at 04:49:47PM +0100, Catalin Marinas wrote:
> > On Mon, May 22, 2017 at 02:21:00PM +0000, Jayachandran C wrote:
> > > On Mon, May 22, 2017 at 01:22:37PM +0100, Will Deacon wrote:
> > > > 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.
> > > 
> > > 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.
> > 
> > I agree, memremap() is a better way but this was merged in 4.3 while
> > ioremap_wc() had been around for a long time. If you want to change its
> > semantics, you'd need to go through all its uses in the kernel and make
> > sure they *only* use I/O accessors with such memory. At a quick grep,
> > there are several places where the __iomem pointer attribute is dropped
> > shortly after ioremap() and the pointer is accessed directly. That's
> > where things will break with Device GRE memory since the compiler
> > doesn't guarantee aligned accesses.
> 
> There is a difference in behavior with regard to unaligned access between
> ioremap_wc and ioremap already. I don't follow why unaligned access has to
> supported for ioremap_wc but not for ioremap - it would be much better to
> be consistent here.

Please re-read my paragraph above. It's nice if both ioremap_wc() and
ioremap() returned Device memory as long as you fix the ioremap_wc()
misuses throughout the kernel.

> > > Also, patch 1/2 should be useful anyway - can that be picked up?
> > 
> > The patch is harmless but are we going to have a user of Device_GRE?
> 
> This patch would make the attribute in MAIR and the index MT_DEVICE_GRE
> useful for __ioremap. Otherwise I don't see the use in having the
> attribute at  all. I can at least give this to people who want Device GRE
> mapping, which is the fastest way to access device memory on ThunderX2.

Is this a driver that's going to be used on arm64 kernels only? I'm not
that keen on __ioremap() since it's not a standard driver API, so you
end up with #ifdef CONFIG_ARM64 in the driver code.

-- 
Catalin