From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D77E9C2BA2B for ; Fri, 17 Apr 2020 02:09:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A816E221F9 for ; Fri, 17 Apr 2020 02:09:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="hd+9RLdO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728989AbgDQCJz (ORCPT ); Thu, 16 Apr 2020 22:09:55 -0400 Received: from perceval.ideasonboard.com ([213.167.242.64]:46610 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728969AbgDQCJz (ORCPT ); Thu, 16 Apr 2020 22:09:55 -0400 Received: from pendragon.ideasonboard.com (81-175-216-236.bb.dnainternet.fi [81.175.216.236]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id 074C697D; Fri, 17 Apr 2020 04:09:51 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1587089392; bh=aOJYR1rkQxzhPfNAEcrk20mLkVZECzCoRj7kVov3h0Y=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hd+9RLdO3EIQiOjyOzIy3SGshr4Ot8mh0XQvXBVdxQdblIrY9LocV6mgBMEsq7mQj GFiH8t7FF/FgZp0G0wovnbPJEYSd0EyKD+0lmgVrWHcIQIloACo5iJSUXxmxL3c+/x LKMnXLpw7eBR/F4/bxhB022k91DSsdwgwrqC2TCE= Date: Fri, 17 Apr 2020 05:09:39 +0300 From: Laurent Pinchart To: Sakari Ailus Cc: Lad Prabhakar , Mauro Carvalho Chehab , Rob Herring , Mark Rutland , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , NXP Linux Team , Kieran Bingham , Geert Uytterhoeven , linux-media@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Lad Prabhakar , Maxime Ripard Subject: Re: [PATCH v5 2/5] media: i2c: ov5645: Drop reading clock-frequency dt-property Message-ID: <20200417020939.GI28162@pendragon.ideasonboard.com> References: <1586191361-16598-1-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com> <1586191361-16598-3-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com> <20200406165108.GA7646@kekkonen.localdomain> <20200406173234.GD16885@pendragon.ideasonboard.com> <20200407062241.GA8883@kekkonen.localdomain> <20200407122106.GD4751@pendragon.ideasonboard.com> <20200407151401.GA5206@paasikivi.fi.intel.com> <20200414205552.GN19819@pendragon.ideasonboard.com> <20200415162722.GG27762@paasikivi.fi.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200415162722.GG27762@paasikivi.fi.intel.com> Sender: devicetree-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org Hi Sakari, On Wed, Apr 15, 2020 at 07:27:22PM +0300, Sakari Ailus wrote: > On Tue, Apr 14, 2020 at 11:55:52PM +0300, Laurent Pinchart wrote: > > On Tue, Apr 07, 2020 at 06:14:01PM +0300, Sakari Ailus wrote: > >> On Tue, Apr 07, 2020 at 03:21:06PM +0300, Laurent Pinchart wrote: > >>> On Tue, Apr 07, 2020 at 09:22:41AM +0300, Sakari Ailus wrote: > >>>> On Mon, Apr 06, 2020 at 08:32:34PM +0300, Laurent Pinchart wrote: > >>>>> On Mon, Apr 06, 2020 at 07:51:08PM +0300, Sakari Ailus wrote: > >>>>>> On Mon, Apr 06, 2020 at 05:42:38PM +0100, Lad Prabhakar wrote: > >>>>>>> Modes in the driver are based on xvclk frequency fixed to 24MHz, but where > >>>>>>> as the OV5645 sensor can support the xvclk frequency ranging from 6MHz to > >>>>>>> 24MHz. So instead making clock-frequency as dt-property just let the > >>>>>>> driver enforce the required clock frequency. > >>>>>> > >>>>>> Even if some current systems where the driver is used are using 24 MHz > >>>>>> clock, that doesn't mean there wouldn't be systems using another frequency > >>>>>> that the driver does not support right now. > >>>>>> > >>>>>> The driver really should not set the frequency unless it gets it from DT, > >>>>>> but I think the preferred means is to use assigned-clock-rates instead, and > >>>>>> not to involve the driver with setting the frequency. > >>>>>> > >>>>>> Otherwise we'll make it impossible to support other frequencies, at least > >>>>>> without more or less random defaults. > >>>>> > >>>>> We're running in circles here. > >>>>> > >>>>> As the driver only supports 24MHz at the moment, the frequency should be > >>>>> set by the driver, as it's a driver limitation. We can then work on > >>>>> supporting additional frequencies, which will require DT to provide a > >>>>> list of supported frequencies for the system, but that can be done on > >>>>> top. > >>>> > >>>> I guess it would be possible to use different external clock frequencies on > >>>> a sensor in a given system but that seems to be a bit far fetched, to the > >>>> extent I've never seen anyone doing that in practice. > >>>> > >>>> Originally, the driver set the frequency based on the clock-frequency > >>>> property. If we're removing that but use a fixed frequency instead, then > >>>> how is that going to work going forward when someone adds support for other > >>>> frequencies in the driver and has a system requiring that, while there are > >>>> some other platforms relying on the driver setting a particular frequency? > >>> > >>> The standard property for this is link-frequencies, not clock-frequency. > >>> Deprecating clock-frequency now paves the way to use the standard > >>> property later when/if someone implements support for additional > >>> frequencies. > >> > >> The external clock frequency and link frequency are different indeed, but > >> they are related. The link frequency has been selected in a way that it is > >> possible to generate that exact frequency using the chosen external clock > >> frequency. If you change the external clock frequency, chances are good > >> there is no PLL configuration to generate that link frequency. > > > > But aren't we supposed to pick the clock frequency based on the link > > frequency specified in DT ? > > No. In a general case there is no reliable way to come up with an external > clock frequency based on another, different if related, frequency. > > > In any case, this policy needs to be carefully documented. > > I thought after ten or so years this would be already an established > practice. :-) > > I agree it should be documented. We don't seem to have specific > documentation for camera sensor drivers at the moment. I can submit a > patch... > > >>>> Although, if you're saying that this driver only needs to work with DT that > >>>> comes with the kernel and you don't care about DT binary compatibility, > >>>> this would be fine. > >>> > >>> I believe this series to not break backward compatibility, as the driver > >>> only works with a 24MHz clock, so I expect all DTs to specify that. > >> > >> What you're still doing here is defining the DT bindings based on the > >> current driver implementation, not the device properties. > > > > Quite the contrary, the device doesn't require any particular input > > clock frequency, so we're removing that from DT :-) Specifying the clock > > frequency in DT is in my opinion a manual workaround for not computing > > it at runtime based on the desired link frequency, while the link > > frequency is a property of the system as it specifies the range of link > > frequencies that are safe to use from an EMC point of view. > > The external clock frequency is significantly lower than the link frequency > (usually), but it still comes out of the SoC (or a PMIC chip). The clock > signal track on PCB as well as wiring may also be rather long, depending on > where the camera sensor is --- quite possibly tens of centimetres. > Therefore I wouldn't categorically rule out possible EMC issues with that > one either. That's a valid point. > The bottom line is: use a known-good, safe frequency. What if different input clock frequencies are needed to achieve different link frequencies ? -- Regards, Laurent Pinchart