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

[Lad, Prabhakar]

Hi Conor,

On Fri, Nov 25, 2022 at 12:25 PM Conor Dooley
<conor.dooley@xxxxxxxxxxxxx> 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;
        };
        andestech,pma-region@0xdeadbeef {
            reg = <0xdeadbeef 0x08000000>;
            andestech,mem-non-cacheable-bufferable;
        };
        ....
    };

Did I chime in this time?

Cheers,
Prabhakar