linux-kernel - Re: [net-next PATCH v14 07/16] net: mdio: regmap: add support for C45 read/write

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Message-ID: <20250409090751.6bc42b5b@fedora.home>
Date: Wed, 9 Apr 2025 09:07:51 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Christian Marangi <ansuelsmth@...il.com>
Cc: Lee Jones <lee@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof
 Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Andrew
 Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, Vladimir Oltean <olteanv@...il.com>, Srinivas
 Kandagatla <srinivas.kandagatla@...aro.org>, Heiner Kallweit
 <hkallweit1@...il.com>, Russell King <linux@...linux.org.uk>, "Chester A.
 Unal" <chester.a.unal@...nc9.com>, Daniel Golle <daniel@...rotopia.org>,
 DENG Qingfang <dqfext@...il.com>, Sean Wang <sean.wang@...iatek.com>, Simon
 Horman <horms@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org,
 netdev@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, upstream@...oha.com
Subject: Re: [net-next PATCH v14 07/16] net: mdio: regmap: add support for
 C45 read/write

Hi Christian,

On Tue,  8 Apr 2025 11:51:14 +0200
Christian Marangi <ansuelsmth@...il.com> wrote:

> Add support for C45 read/write for mdio regmap. This can be done
> by enabling the support_encoded_addr bool in mdio regmap config and by
> using the new API devm_mdio_regmap_init to init a regmap.
> 
> To support C45, additional info needs to be appended to the regmap
> address passed to regmap OPs.
> 
> The logic applied to the regmap address value:
> - First the regnum value (20, 16)
> - Second the devnum value (25, 21)
> - A bit to signal if it's C45 (26)
> 
> devm_mdio_regmap_init MUST be used to register a regmap for this to
> correctly handle internally the encode/decode of the address.
> 
> Drivers needs to define a mdio_regmap_init_config where an optional regmap
> name can be defined and MUST define C22 OPs (mdio_read/write).
> To support C45 operation also C45 OPs (mdio_read/write_c45).
> 
> The regmap from devm_mdio_regmap_init will internally decode the encoded
> regmap address and extract the various info (addr, devnum if C45 and
> regnum). It will then call the related OP and pass the extracted values to
> the function.
> 
> Example for a C45 read operation:
> - With an encoded address with C45 bit enabled, it will call the
>   .mdio_read_c45 and addr, devnum and regnum will be passed.
>   .mdio_read_c45 will then return the val and val will be stored in the
>   regmap_read pointer and will return 0. If .mdio_read_c45 returns
>   any error, then the regmap_read will return such error.
> 
> With support_encoded_addr enabled, also C22 will encode the address in
> the regmap address and .mdio_read/write will called accordingly similar
> to C45 operation.

This driver's orginal goal is to address the case where we have a
PHY-like device that has the same register layout and behaviour as a
C22 PHY, but where the registers are not accesses through MDIO (MMIO
for example, as in altera-tse or dwmac-socfpga, or potentially SPI even
though  there's no example upstream).

What is done here is quite different, I guess it could work if we have
MMIO C45 phys that understand the proposed encoding, but I don't really
understand the dance where C45 accesses are wrapped by this mdio-regmap
driver into regmap accesss, but the regmap itself converts it back to
C45 accesses. Is it just so that it fits well with MFD ?

I'm not really against that, it still converts mdio access to regmap so
there's that, but is there a way to elaborate or document somewhere why
we need to do go through C45 -> regmap -> C45 instead of just
writing a mii_bus driver in the first place ?

As I said, I think this could work and even be re-used in other places,
so I'm ok with that, it's just not really clear from the commit log what
problem this solves.

Maxime