Re: [RFC v2 4/4] ARM: keystone: dma-coherent with safe fallback

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jun 06, 2016 at 12:43:21PM +0100, Russell King - ARM Linux wrote:
> On Mon, Jun 06, 2016 at 09:56:27AM +0100, Mark Rutland wrote:
> > I very much do not like this. As I previously mentioned [1],
> > dma-coherent has de-facto semantics today. This series deliberately
> > changes that, and inverts the relationship between DT and kernel (as the
> > describption in the DT would now depend on the configuration of the
> > kernel).
> 
> dma-coherent's semantics are not very well defined - just grep for it
> in Documention/devicetree/ and you'll find several different wordings
> for what this property means.

Indeed. This is the tip of the iceberg w.r.t. under-specification of
memory attribute usage.

> Anyway, my point here is that all of these merely say that the hardware
> is coherent in _some regard_ - it doesn't specify under what conditions
> DMA coherency is guaranteed by the hardware.  It happens that on ARM,
> most platforms give that guarantee when using inner shared mappings.  If
> we were to use some other sharing, or disable sharing altogether (eg, by
> disabling SMP support) then all these platforms would immediately break.
> 
> In other words, DMA coherence today already depends on the kernel's setup
> of the page tables corresponding to the requirements of the hardware.

I agree that whether or not devices are coherent in practice depends on
the kernel's configuration. The flip side, as you point out, is that
devices are coherent when a specific set of attributes are used.

i.e. that if you read dma-coherent as meaning "coherent iff Normal,
Inner Shareable, Inner WB Cacheable, Outer WB Cacheable", then
dma-coherent consistently describes the same thing, rather than
depending on the configuration of the OS.

DT is a datastructure provided to the kernel, potentially without deep
internal knowledge of that kernel configuration. Having a consistent
rule that is independent of the kernel configuration seems worth aiming
for.

A dma-outer-coherent property would allow us to accurately describe the
keystone case in the same way, independent of kernel configuration.

> Keystone II is just slightly different - and as I understand it, TI
> followed one of the early specifications that ARM Ltd produced.  That
> specification may have contained errors, but unfortunately, we now have
> a situation where there is hardware out there which followed in good
> faith.

To be clear, I am not arguing against supporting keystone. I just wish
to avoid muddying the waters further w.r.t. the semantics of
dma-coherent, which I believe can be salvaged and made consistent.

Clearly, those semantics are the point of contention here.

Thanks,
Mark.