linux-kernel - Re: [PATCH net-next v5 2/6] net: dsa: microchip: Add KSZ8463 switch support to KSZ DSA driver

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Message-ID: <857db988-fd4d-4cac-9774-43161f1f592a@lunn.ch>
Date: Wed, 23 Jul 2025 19:16:01 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Tristram.Ha@...rochip.com
Cc: Woojung Huh <woojung.huh@...rochip.com>,
	Vladimir Oltean <olteanv@...il.com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Simon Horman <horms@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Marek Vasut <marex@...x.de>, UNGLinuxDriver@...rochip.com,
	devicetree@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v5 2/6] net: dsa: microchip: Add KSZ8463 switch
 support to KSZ DSA driver

> KSZ8463 switch is a 3-port switch based from KSZ8863.  Its major
> difference from other KSZ SPI switches is its register access is not a
> simple continual 8-bit transfer with automatic address increase but uses
> a byte-enable mechanism specifying 8-bit, 16-bit, or 32-bit access.  Its
> registers are also defined in 16-bit format because it shares a design
> with a MAC controller using 16-bit access.  As a result some common
> register accesses need to be re-arranged.  The 64-bit access used by
> other switches needs to be broken into 2 32-bit accesses.

> +	if (ksz_is_ksz8463(dev)) {
> +		int i;
> +
> +		for (i = 0; i < 2; i++)
> +			ret = regmap_read(ksz_regmap_32(dev), reg + i * 4,
> +					  &value[i]);
> +		*val = (u64)value[0] << 32 | value[1];
> +		return ret;
> +	}
>  	ret = regmap_bulk_read(ksz_regmap_32(dev), reg, value, 2);

This needs a bit more explanation. When i look at

https://elixir.bootlin.com/linux/v6.15.7/source/drivers/base/regmap/regmap.c#L3117

It appears to do something similar, looping over count doing
_regmap_read().

There is also:

https://elixir.bootlin.com/linux/v6.15.7/source/include/linux/regmap.h#L370

 * @use_single_read: If set, converts the bulk read operation into a series of
 *                   single read operations. This is useful for a device that
 *                   does not support  bulk read.
 * @use_single_write: If set, converts the bulk write operation into a series of
 *                    single write operations. This is useful for a device that
 *                    does not support bulk write.

It would be better if regmap_bulk_read() could be made to work, and
hide away the differences, which is what regmap is all about.

	Andrew