Re: [PATCH 0/4] MIPS: Ordering enforcement fixes for MMIO accessors

[Paul Burton]

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