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SubjectRe: [PATCH 0/4] MIPS: Ordering enforcement fixes for MMIO accessors
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Hi Maciej,

On Mon, Oct 08, 2018 at 01:36:55AM +0100, Maciej W. Rozycki wrote:
> This patch series is a follow-up to my earlier consideration about MMIO
> access ordering recorded here: <https://lkml.org/lkml/2014/4/28/201>.
>
> As I have learnt in a recent Alpha/Linux discussion starting here:
> <https://marc.info/?i=alpine.LRH.2.02.1808161556450.13597%20()%20file01%20!%20intranet%20!%20prod%20!%20int%20!%20rdu2%20!%20redhat%20!%20com>
> related to MMIO accessor ordering barriers ports are actually required to
> follow the x86 strongly ordered semantics. As the ordering is not
> specified in the MIPS architecture except for the SYNC instruction we do
> have to put explicit barriers in MMIO accessors as otherwise ordering may
> not be guaranteed.
>
> Fortunately on strongly ordered systems SYNC is expected to be as cheap
> as a NOP, and on weakly ordered ones it is needed anyway. As from
> revision 2.60 of the MIPS architecture specification however we have a
> number of SYNC operations defined, and SYNC 0 has been upgraded from an
> ordering to a completion barrier. We currently don't make use of these
> extra operations and always use SYNC 0 instead, which this means that we
> may be doing too much synchronisation with the barriers we have already
> defined.
>
> This patch series does not make an attempt to optimise for SYNC operation
> use, which belongs to a separate improvement. Instead it focuses on
> fixing MMIO accesses so that drivers can rely on our own API definition.

Agreed, using the lightweight sync types is a whole other can of worms.
I did speak with the architecture team about the description of SYNC
recently (in the context of nanoMIPS documentation if I recall) and hope
the tweaks that were made to the architectural description of it might
help with using them one day soon.

> Following the original consideration specific MMIO barrier operations are
> added. As they have turned out to be required to be implied by MMIO
> accessors there is no immediate need to make them form a generic
> cross-architecture internal Linux API. Therefore I defined them for the
> MIPS architecture only, using the names originally coined by mostly taking
> them from the PowerPC port.
>
> Then I have used them to fix `mmiowb', and then `readX' and `writeX'
> accessors. Finally I have updated the `_relaxed' accessors to avoid
> unnecessary synchronisation WRT DMA.

Thanks - this definitely leaves us in a better place than we were :)

All 4 patches applied to mips-next for 4.20.

Paul

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