From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benoit Cousson Subject: Re: [PATCH v3 1/2] rtc: omap: update of_device_id to reflect latest ip revisions Date: Fri, 16 Aug 2013 20:12:46 +0200 Message-ID: <520E6B9E.5080706@baylibre.com> References: <1376653017-21935-1-git-send-email-gururaja.hebbar@ti.com> <1376653017-21935-2-git-send-email-gururaja.hebbar@ti.com> <520E341D.4080206@baylibre.com> <20130816172022.GA3719@e106331-lin.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-we0-f170.google.com ([74.125.82.170]:50819 "EHLO mail-we0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753076Ab3HPSMu (ORCPT ); Fri, 16 Aug 2013 14:12:50 -0400 Received: by mail-we0-f170.google.com with SMTP id w60so1965928wes.29 for ; Fri, 16 Aug 2013 11:12:49 -0700 (PDT) In-Reply-To: <20130816172022.GA3719@e106331-lin.cambridge.arm.com> Sender: linux-omap-owner@vger.kernel.org List-Id: linux-omap@vger.kernel.org To: Mark Rutland Cc: "Hebbar, Gururaja" , "akpm@linux-foundation.org" , "khilman@linaro.org" , "tony@atomide.com" , "rob.herring@calxeda.com" , "a.zummo@towertech.it" , "rob@landley.net" , "grant.likely@linaro.org" , "rtc-linux@googlegroups.com" , "linux-omap@vger.kernel.org" , "devicetree@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "davinci-linux-open-source@linux.davincidsp.com" , "sudhakar.raj@ti.com" Hi Mark, On 16/08/2013 19:20, Mark Rutland wrote: > Hi Benoit, > > On Fri, Aug 16, 2013 at 03:15:57PM +0100, Benoit Cousson wrote: >> Hi Gururaja, >> >> On 16/08/2013 13:36, Hebbar, Gururaja wrote: >>> The syntax of compatible property in DT is to mention the Most specific >>> match to most generic match. >>> >>> Since AM335x is the platform with latest IP revision, add it 1st in >>> the device id table. >> >> I don't understand why? The order should not matter at all. >> >> I've tried to follow the thread you had with Mark on the v2, but AFAIK, >> you've never answered to his latest question. >> >> Moreover, checking the differences between the Davinci and the am3352 >> RTC IP, I would not claim that both are compatible. >> >> Sure you can use the am3352 with the Davinci driver, but you will lose >> the wakeup functionality without even being notify about that. > > Could you describe the wakeup functionality, and how it differs between > the am3352-rtc and the da830-rtc? AFAIK, da830-rtc does not have that functionality at all. This is something that was added to the am3352-rtc. > As I understand it, the am3352 functionality is a superset of the da830 > functionality. You can use the old driver, and get some functionality, > or use the new driver and get it all. Mmm, what your are saying now seems to make sense to me as well. So I'm even more confused :-) > That means that am3352-rtc is compatible with da830. As long as the > kernel first checks for am3352-rtc, there will be *no* loss of > functionality. All this does is enable older kernels to use the hardware > in some fashion, and given the older kernel didn't have support for the > am3352-rtc features, this is a *gain* in functionality, not a loss. > >> >> For my point of view, compatible mean that the HW will still be fully >> functional with both versions of the driver, which is not the case here. > > What? A driver for any entry in the compatible list should be able to > drive the hardware to *some* level of functionality. We list from > most-specific to most-general to allow a graceful degradation from fully > supported to bare minimum functionality. OK, but where is it written in the DT spec that this is what the compatible is supposed to mean? I'm quoting it again: " The compatible property value consists of one or more strings that define the specific programming model for the device. This list of strings should be used by a client program for device driver selection. The property value consists of a concatenated list of null terminated strings, from most specific to most general. They allow a device to express its compatibility with a family of similar devices, potentially allowing a single device driver to match against several devices. " The graceful degradation or the loss of functionality is not something that I really understand in that text. Anyway, I'm probably too tired... I'll go back home, and think about that after the week-end. Regards, Benoit