From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from metis.ext.pengutronix.de ([2001:67c:670:201:290:27ff:fe1d:cc33]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kNbvU-0007gF-Oj for barebox@lists.infradead.org; Wed, 30 Sep 2020 13:13:45 +0000 References: <20200928154247.15619-1-a.fatoum@pengutronix.de> <20200929073220.GU11648@pengutronix.de> <50c46e25-4b69-2868-e264-72b37b82244e@pengutronix.de> <20200930074847.GH11648@pengutronix.de> From: Ahmad Fatoum Message-ID: Date: Wed, 30 Sep 2020 15:13:42 +0200 MIME-Version: 1.0 In-Reply-To: <20200930074847.GH11648@pengutronix.de> Content-Language: en-US List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "barebox" Errors-To: barebox-bounces+u.kleine-koenig=pengutronix.de@lists.infradead.org Subject: Re: [PATCH 1/7] driver: introduce less error-prone dev_get_drvdata alternative To: Sascha Hauer Cc: barebox@lists.infradead.org Hi, On 9/30/20 9:48 AM, Sascha Hauer wrote: >> If the match data is a valid pointer: >> -> It doesn't matter, why we have no match data either way. >> >> If the match data is a casted integer (e.g. enum): >> The driver author should either: >> -> place the default enum value as first one, >> so no match data => default > > You know that, but "The driver Author" probably doesn't. Like with all other functions that return an error code, the author should check those errors: enum lm75_type type = (enum lm75_type)device_get_match_data(dev); if (type == 0) return -ENODEV; I'd expect they will see that 0 shouldn't be part of the enumeration. >> -> should add an initial DEVICE_TYPE_UNKNOWN = 0 in the enum >> and handle it appropriately >> >> I like the function signature like that, I don't really see >> a need to adjust it. >> >>> As you realize in your series some drivers cast the returned value into >>> an integer type and use 0 as a valid possibility. These need an extra >>> explanation why we can accept that case. I can think of different >>> possibilies to get that straight: >>> >>> - Put a real pointer into matchdata. This is really preferred as it >>> motivates people to put flags into a (struct type) matchdata which >>> doesn't lead to excessive if (type == foo || type == bar || type == >>> baz) we sometimes see in drivers. >> >> We have a real pointer there already. The problem is migrating the >> existing drivers. > > Yes, existing drivers would have to be migrated, that is exactly what I > am proposing. > >> >>> - Return an ERR_PTR from device_get_match_data(). this is less likely >>> interpreted as a valid int value >> >> Doesn't cover all cases. Also for the normal use, it means >> you need to have to check with IS_ERR_OR_NULL everywhere to >> be sure you don't dereference a NULL pointer. > > Why that? Just don't return NULL when there's no match data, but return > -ESOMETHING. For the lm75 we would have to do: enum lm75_type { TYPE_1, TYPE_2 }; const void *match = device_get_match_data(dev); if (IS_ERR(match)) return PTR_ERR(match); enum lm75_type type = (enum lm75_type)match; The alternative being: enum lm75_type { TYPE_UNKNOWN = 0, TYPE_1, TYPE_2 }; enum lm75_type type = (enum lm75_type)device_get_match_data(dev); if (type == TYPE_UNKNOWN) return -ENODEV; I prefer the second one very much. > > Sascha > -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@lists.infradead.org http://lists.infradead.org/mailman/listinfo/barebox