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

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Message-ID: <CAHQ1cqGN6fbbXoqZaFpDG856EsHAve-Or0M_HT8Zpb-BodC3sA@mail.gmail.com>
Date:	Tue, 19 Jul 2016 11:11:54 -0700
From:	Andrey Smirnov <andrew.smirnov@...il.com>
To:	Richard Weinberger <richard@....at>
Cc:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	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, 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?

Andrey