linux-kernel - Re: Question: dev_err

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Message-ID: <Yu9id1O98e6or1qm@kroah.com>
Date:   Sun, 7 Aug 2022 08:57:59 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Janitors <kernel-janitors@...r.kernel.org>
Subject: Re: Question: dev_err_probe() vs Printk Index

On Sat, Aug 06, 2022 at 10:53:16AM +0200, Christophe JAILLET wrote:
> Hi,
> 
> When a driver is using dev_err(), part of it is inlined and it:
>    - takes advantage of dev_fmt()  [1]
>    - implements Printk Index       [2]
> 
> Printk Index works with some __builtin_constant_p() magic in it.
> In case of a use in a probe, 99.99% of the time the log level and the format
> will be constant and the logic for Printk Index will be put in place.
> 
> 
> In case dev_err_probe(), the format will be an argument passed to the
> function and will not be constant, so nothing will be generated in the
> 'printk'_index section.
> 
> 
> In case dev_err_probe(), a potential dev_fmt() defined in the drivers' file
> can't be taken into consideration.
> (trusting my grep, we never use in files that define dev_fmt() in the .c
> file. I've not checked if it is true via #include "<something.h>")
> 
> 
> Even if I've read [3], I don't fully understand the real need of this Printk
> Index mechanism (at least for my own needs :))
> 
> 
> My questions are:
>    - is my analysis right?
>    - is the lack of these 2 functionalities (dev_fmt and Printk Index)
> expected, when dev_err_probe() is used?
>    - if not, is it a issue?
>    - should it be at least documented?

The printk index stuff is odd, and always seemed like a "check box"
option that some people wanted for a niche enterprise market.  It's up
to them to keep that working well if they really need it, driver authors
should not worry about this.

thanks,

greg k-h