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

[Ivan Vecera <ivecera@redhat.com>]

On 24. 04. 25 9:57 odp., Andrew Lunn wrote:
> 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.

Just note that some of 48bit registers uses signed values so the check
should be:

x is in <S48_MIN, U48_MAX>

this could be done using bit operations:

(!(GENMASK_ULL(63, 49) & ((u64)(x) + BIT_ULL(47))))

Ivan