netdev - Re: [RFC PATCH V2 net-next 03/11] net: hibmcge: Add mdio and hardware configuration supported in this module

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Message-ID: <ef78137e-d977-4da5-acfb-00865e3a9837@huawei.com>
Date: Sat, 17 Aug 2024 09:11:24 +0800
From: Jijie Shao <shaojijie@...wei.com>
To: Andrew Lunn <andrew@...n.ch>
CC: <shaojijie@...wei.com>, <yisen.zhuang@...wei.com>,
	<salil.mehta@...wei.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <horms@...nel.org>,
	<shenjian15@...wei.com>, <wangpeiyang1@...wei.com>, <liuyonglong@...wei.com>,
	<sudongming1@...wei.com>, <xujunsheng@...wei.com>, <shiyongbang@...wei.com>,
	<jdamato@...tly.com>, <jonathan.cameron@...wei.com>,
	<shameerali.kolothum.thodi@...wei.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH V2 net-next 03/11] net: hibmcge: Add mdio and hardware
 configuration supported in this module


on 2024/8/17 5:04, Andrew Lunn wrote:
> On Fri, Aug 16, 2024 at 02:10:36PM +0800, Jijie Shao wrote:
>> on 2024/8/16 10:25, Andrew Lunn wrote:
>>>> +struct hbg_mdio_command {
>>>> +	union {
>>>> +		struct {
>>>> +			u32 mdio_devad : 5;
>>>> +			u32 mdio_prtad :5;
>>>> +			u32 mdio_op : 2;
>>>> +			u32 mdio_st : 2;
>>>> +			u32 mdio_start : 1;
>>>> +			u32 mdio_clk_sel : 1;
>>>> +			u32 mdio_auto_scan : 1;
>>>> +			u32 mdio_clk_sel_exp : 1;
>>>> +			u32 rev : 14;
>>>> +		};
>>>> +		u32 bits;
>>>> +	};
>>>> +};
>>> This is generally not the way to do this. Please look at the macros in
>>> include/linux/bitfield.h. FIELD_PREP, GENMASK, BIT, FIELD_GET
>>> etc. These are guaranteed to work for both big and little endian, and
>>> you avoid issues where the compiler decides to add padding in your
>>> bitfields.
>>>
>>> 	Andrew
>> Thanks, I already know about macros like FIELD_PREP or FIELD_GET.
>> and these macros are already used in parts of this patch set.
>>
>> But I think this writing style in here very convenient.
>> Although padding needs to be added during definition,
>> but you can use command.mdio_start or command->mdio_start
>> to access specific bitfields.
>>
>> Although FIELD_PREP/FIELD_GET is convenient,
>> But it also needs to define the mask using BIT or GENMASK,
>> and the mask can only be a static constant,
>> which makes it difficult to use sometimes.
> Have a look around. How many drivers use this sort of union? How many
> use bitfield.h. There is a reason the union is not used. I suspect
> there is nothing in the C standard which guarantees it works.
>
>        Andrew

Ok, thanks for the comment.
I'll fix it in the next version.
	
	Jijie Shao