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

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Date:   Tue, 4 Jul 2023 17:52:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Guenter Roeck <linux@...ck-us.net>
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 Tue, Jul 04, 2023 at 09:43:39AM -0700, Guenter Roeck wrote:
> 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 ?

Nothing, I'm missing it, sorry.  Been a long day, let me redo this...

greg k-h