linux-kernel - Re: [PATCH 1/2] mtd: nand: BUG_ON in case of no select_chip and cmd

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Date:	Tue, 19 Jul 2016 20:16:11 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Andrey Smirnov <andrew.smirnov@...il.com>
Cc:	Richard Weinberger <richard@....at>, linux-mtd@...ts.infradead.org,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mtd: nand: BUG_ON in case of no select_chip and
 cmd_ctrl

On Tue, 19 Jul 2016 11:11:54 -0700
Andrey Smirnov <andrew.smirnov@...il.com> wrote:

> On Tue, Jul 19, 2016 at 9:22 AM, Richard Weinberger <richard@....at> wrote:
> > Am 19.07.2016 um 18:12 schrieb Boris Brezillon:  
> >>>> Not sure a BUG_ON() is worst than a NULL-pointer exception ;-).  
> >>>
> >>> When this really just triggers a NULL-pointer exception, we don't need a BUG_ON or WARN_ON at
> >>> all since the kernel can tell anyway what went wrong.  
> >>
> >> Hm, that's not entirely true, depending on your debug options you don't
> >> have all the information to guess which line triggered the NULL pointer
> >> exception, and this makes it harder to debug.
> >> And I agree with Andrey here, it's better to complain at registration
> >> time than letting the controller register all its NAND devices and
> >> generate exceptions when the NAND is really used.
> >>
> >> BTW, I don't quite understand the rational behind BUG_ON() eradication.
> >> I agree that they should not be used when the driver can recover from a
> >> specific failure, but that's not really the case here (some NAND
> >> controller drivers don't check nand_scan_tail() or nand_scan() return
> >> code).  
> >
> > I've been told that new code (except core code) should not BUG()/_ON().
> >  
> >> The best solution would probably be to patch all those drivers and then
> >> return an error when one of the mandatory hooks is missing, but in the
> >> meantime I don't see any problem in adding BUG_ON() calls.  
> >
> > Yes, definitely.  
> 
> I don't have any preferences as far BUG_ON/WARN_ON are concerned and
> am more than happy to change one for another.
> 
> The reason I came up with that patch is that I stumbled on that
> segfault (by not providing custom select_chip() and not setting up
> cmd_ctrl()) and it took me good 20 minutes to figure out the nature of
> the problem, whereas, IMHO, having a BUG/WARN statement at the would
> have been more self-documenting/explanatory.
> 
> What if I modify the patch to change nand_set_default's signature to
> return a error code, add corresponding checking in
> nand_get_flash_type()/nand_scan_ident() and replace BUG_ON with
> WARN_ON? Would it be more agreeable solution?

Agreed.