linux-kernel - Re: [PATCH net-next v4 5/8] mfd: zl3073x: Add functions to work with register mailboxes

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Message-ID: <34f29b17-dc68-4005-b2da-95fde34117a0@lunn.ch>
Date: Thu, 24 Apr 2025 21:57:05 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Ivan Vecera <ivecera@...hat.com>
Cc: netdev@...r.kernel.org, Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Arkadiusz Kubalewski <arkadiusz.kubalewski@...el.com>,
	Jiri Pirko <jiri@...nulli.us>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Prathosh Satish <Prathosh.Satish@...rochip.com>,
	Lee Jones <lee@...nel.org>, Kees Cook <kees@...nel.org>,
	Andy Shevchenko <andy@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Schmidt <mschmidt@...hat.com>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH net-next v4 5/8] mfd: zl3073x: Add functions to work with
 register mailboxes

On Thu, Apr 24, 2025 at 09:53:39PM +0200, Ivan Vecera wrote:
> 
> 
> On 24. 04. 25 9:18 odp., Andrew Lunn wrote:
> > > During taking 613cbb91e9ce ("media: Add MIPI CCI register access helper
> > > functions") approach I found they are using for these functions u64
> > > regardless of register size... Just to accommodate the biggest
> > > possible value. I know about weakness of 'void *' usage but u64 is not
> > > also ideal as the caller is forced to pass always 8 bytes for reading
> > > and forced to reserve 8 bytes for each read value on stack.
> > 
> > In this device, how are the u48s used? Are they actually u48s, or are
> > they just u8[6], for example a MAC address? The network stack has lots
> > of functions like:
> > 
> > eth_hw_addr_set(struct net_device *dev, const u8 *addr)
> 
> u48 registers always represent 48bit integer... they read from device using
> bulk read as big-endian 48bit int. The same is valid also for u16
> and u32.

Then a u64 makes sense, plus on write to hardware a check the upper
bits are 0. These u48s are going to be stored in a u64 anyway, since C
does not have a u48 type.

But all the other types do exist in C, so you should use them and have
type checking.

	Andrew