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 16:55:11 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Conor Dooley <conor.dooley@...rochip.com>
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 25/11/2022 13:25, Conor Dooley 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:

I did not comment about properties. I comment about constants. Why
register values/offsets/addresses are in this particular case suitable
for binding headers?

> - 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?


Best regards,
Krzysztof