linux-kernel - Re: [PATCH 1/3] hwmon: (oxp-sensors): remove static board variable

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Date:   Tue, 4 Jul 2023 09:43:39 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-hwmon@...r.kernel.org, samsagax@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] hwmon: (oxp-sensors): remove static board variable

On 7/4/23 09:14, Greg Kroah-Hartman wrote:
> On Tue, Jul 04, 2023 at 07:14:54AM -0700, Guenter Roeck wrote:
>> On 7/4/23 06:44, Greg Kroah-Hartman wrote:
>>> On Tue, Jul 04, 2023 at 06:39:07AM -0700, Guenter Roeck wrote:
>>>> On 7/4/23 06:17, Greg Kroah-Hartman wrote:
>>>>> Drivers should not have a single static variable for the type of device
>>>>> they are bound to.  While this driver is really going to only have one
>>>>> device at a time in the system, remove the static variable and instead,
>>>>> look up the device type when needed.
>>>>>
>>>>
>>>> This is expensive. I think it would be much better to just move
>>>> the board type detection into the init code and not instantiate
>>>> the driver in the fist place if the board type is unknown.
>>>
>>> The board type detection is all over the place in the driver, it's not
>>> just for "unknown" types, so how about just saving the board type at
>>> probe time and using it then for all other places?
>>>
>>
>> I must be missing something. The current code detects the board type
>> only once, in the probe function. Otherwise the static variable is used.
>> You are replacing it with repeated calls to get_board_type().
>> The whole point of the static variable is to avoid the cost of repeated
>> calls to dmi_first_match().
> 
> Ah, ok, yes, I was refering to the fact that the driver relies on the
> detection of the device type in lots of different places (and doesn't
> ever error out from the detection call.)
> 

I am lost again. Current code:

         dmi_entry = dmi_first_match(dmi_table);
         if (!dmi_entry || boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
                 return -ENODEV;



>>>> We can handle the static variable separately if it really bothers
>>>> you that much.
>>>
>>> I did this change to make patch 2/3 more "obvious" what is happening
>>> when the in_visible() callback happens, so that you don't have to worry
>>> about the saved value or not.  But this whole patch isn't really needed
>>> if you don't mind the lookup just happening in the in_visible() callback
>>> for the first time.
>>>
>>
>> That would at least be a minimal change, and just add one extra lookup
>> which is only called once (or zero, if it is used to save the board type).
> 
> Ok, I'll switch it up, but really, it's just a simple table lookup loop,
> and none of the detection calls are on a "hot path" that I can
> determine.  Or am I missing something?
> 
>> As I said, my solution would be to move the board type detection
>> into the init function and not instantiate the driver in the first
>> place if the probe function would bail out anyway.
> 
> That's not the case today, the only way the probe function would fail
> today is if the registering of the sysfs files fail.  It does not matter
> if the board detection call passes or not.
> 

Again,
         dmi_entry = dmi_first_match(dmi_table);
         if (!dmi_entry || boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
                 return -ENODEV;
                 ^^^^^^^^^^^^^^^

What am I missing ?

Guenter