Re: [PATCH v4 6/7] dt-bindings: cache: r9a07g043f-l2-cache: Add DT binding documentation for L2 cache controller

[Conor Dooley <conor.dooley@microchip.com>]

On Fri, Nov 25, 2022 at 12:51:34PM +0000, Lad, Prabhakar wrote:
> Hi Conor,
> 
> On Fri, Nov 25, 2022 at 12:25 PM Conor Dooley
> <conor.dooley@microchip.com> wrote:
> >
> > On Fri, Nov 25, 2022 at 01:12:18PM +0100, Krzysztof Kozlowski wrote:
> > > On 25/11/2022 11:34, Lad, Prabhakar wrote:
> > > >>> +/* Device, Non-bufferable */
> > > >>> +#define AX45MP_PMACFG_MTYP_DEV_NON_BUF                       (0 << 2)
> > > >>> +/* Device, bufferable */
> > > >>> +#define AX45MP_PMACFG_MTYP_DEV_BUF                   (1 << 2)
> > > >>> +/* Memory, Non-cacheable, Non-bufferable */
> > > >>> +#define AX45MP_PMACFG_MTYP_MEM_NON_CACHE_NON_BUF     (2 << 2)
> > > >>> +/* Memory, Non-cacheable, Bufferable */
> > > >>> +#define AX45MP_PMACFG_MTYP_MEM_NON_CACHE_BUF         (3 << 2)
> > > >>
> > > >> What are all these? They don't look like flags, because 3 = 1 | 2...
> > > >> they don't look like constants, because we do not use shifts in
> > > >> constants. Are these some register values? I also do not see the header
> > > >> being used in the code, so why having a bindings header if it is not
> > > >> used (DTS is not usage...)?
> > > >>
> > > > These are register bit values for the MTYP[5:2] field. The DTS example
> > > > in the binding doc (above) uses these macros. I haven't included the
> > > > DTS/I patches with this patchset yet do think I should?
> > >
> > > Then why storing it as bindings? Bindings headers describe the interface
> > > implemented by drivers and used by DTS, but this is not implemented by
> > > drivers.
> >
> > IIUC, some of these properties are non-discoverable attributes of the
> > cache controller. I see two things that could be done here that are
> > "better" than #defining bits:
> > - add an RZ/Five specific compatible and use match data to set the
> >   attributes which is only possible if the pma-regions are set on a
> >   per SoC basis
> > - make pma-regions into a child node, in which andestech,non-cacheable
> >   andestech,non-bufferable etc are properties of the child node
> >
> For now the only way to get DMA working without IOCP is to have
> AX45MP_PMACFG_MTYP_MEM_NON_CACHE_BUF. But for future purposes I have
> introduced the other available flags.
> 
> So maybe for now we could just have this flag
> andestech,mem-non-cacheable-bufferable in the binding doc.
> 
>     cache-controller@2010000 {
>         reg = <0x13400000 0x100000>;
>         compatible = "andestech,ax45mp-cache", "cache";
>         interrupts = <508 IRQ_TYPE_LEVEL_HIGH>;
>         cache-line-size = <64>;
>         cache-level = <2>;
>         cache-sets = <1024>;
>         cache-size = <262144>;
>         cache-unified;
>         andestech,pma-region@0x58000000 {
>             reg = <0x58000000 0x08000000>;
>             andestech,mem-non-cacheable-bufferable;

Yah, that's about what I would expect - except splitting the properties
up. I think split up makes more sense from a property description point
of view, rather than needing some sort of
oneOf:
  - non-cacheable-bufferable
  - cacheable-non-bufferable
  - non-cacheable-non-bufferable


>         };
>         andestech,pma-region@0xdeadbeef {
>             reg = <0xdeadbeef 0x08000000>;
>             andestech,mem-non-cacheable-bufferable;
>         };
>         ....
>     };


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