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

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Date:   Fri, 25 Nov 2022 12:25:10 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
CC:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Guo Ren <guoren@...nel.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        Atish Patra <atishp@...osinc.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-riscv@...ts.infradead.org>,
        <linux-renesas-soc@...r.kernel.org>,
        Biju Das <biju.das.jz@...renesas.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v4 6/7] dt-bindings: cache: r9a07g043f-l2-cache: Add DT
 binding documentation for L2 cache controller

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

Prabhakar, does that make sense or am I off with my understanding of the
attributes?

Thanks,
Conor.