Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752330AbbBRQru (ORCPT ); Wed, 18 Feb 2015 11:47:50 -0500 Received: from eusmtp01.atmel.com ([212.144.249.243]:35548 "EHLO eusmtp01.atmel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750985AbbBRQrt (ORCPT ); Wed, 18 Feb 2015 11:47:49 -0500 Date: Wed, 18 Feb 2015 17:47:32 +0100 From: Ludovic Desroches To: Pantelis Antoniou CC: Ludovic Desroches , Mark Rutland , Grant Likely , Matt Porter , Koen Kooi , Guenter Roeck , Rob Herring , Tony Lindgren , Nicolas Ferre , "devicetree@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 2/4] of: DT quirks infrastructure Message-ID: <20150218164732.GH32600@odux.rfo.atmel.com> Mail-Followup-To: Pantelis Antoniou , Mark Rutland , Grant Likely , Matt Porter , Koen Kooi , Guenter Roeck , Rob Herring , Tony Lindgren , Nicolas Ferre , "devicetree@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" References: <1424271576-1952-1-git-send-email-pantelis.antoniou@konsulko.com> <1424271576-1952-3-git-send-email-pantelis.antoniou@konsulko.com> <20150218154106.GC29429@leverpostej> <20150218163238.GG32600@odux.rfo.atmel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 11557 Lines: 272 On Wed, Feb 18, 2015 at 06:39:01PM +0200, Pantelis Antoniou wrote: > Hi Ludovic, > > > On Feb 18, 2015, at 18:32 , Ludovic Desroches wrote: > > > > Hi, > > > > Great something we are waiting for a long time! > > > > On Wed, Feb 18, 2015 at 05:53:50PM +0200, Pantelis Antoniou wrote: > >> Hi Mark, > >> > >>> On Feb 18, 2015, at 17:41 , Mark Rutland wrote: > >>> > >>> Hi Pantelis, > >>> > >>> On Wed, Feb 18, 2015 at 02:59:34PM +0000, Pantelis Antoniou wrote: > >>>> Implement a method of applying DT quirks early in the boot sequence. > >>>> > >>>> A DT quirk is a subtree of the boot DT that can be applied to > >>>> a target in the base DT resulting in a modification of the live > >>>> tree. The format of the quirk nodes is that of a device tree overlay. > >>>> > >>>> For details please refer to Documentation/devicetree/quirks.txt > >>>> > >>>> Signed-off-by: Pantelis Antoniou > >>>> --- > >>>> Documentation/devicetree/quirks.txt | 101 ++++++++++ > >>>> drivers/of/dynamic.c | 358 ++++++++++++++++++++++++++++++++++++ > >>>> include/linux/of.h | 16 ++ > >>>> 3 files changed, 475 insertions(+) > >>>> create mode 100644 Documentation/devicetree/quirks.txt > >>>> > >>>> diff --git a/Documentation/devicetree/quirks.txt b/Documentation/devicetree/quirks.txt > >>>> new file mode 100644 > >>>> index 0000000..789319a > >>>> --- /dev/null > >>>> +++ b/Documentation/devicetree/quirks.txt > >>>> @@ -0,0 +1,101 @@ > >>>> +A Device Tree quirk is the way which allows modification of the > >>>> +boot device tree under the control of a per-platform specific method. > >>>> + > >>>> +Take for instance the case of a board family that comprises of a > >>>> +number of different board revisions, all being incremental changes > >>>> +after an initial release. > >>>> + > >>>> +Since all board revisions must be supported via a single software image > >>>> +the only way to support this scheme is by having a different DTB for each > >>>> +revision with the bootloader selecting which one to use at boot time. > >>> > >>> Not necessarily at boot time. The boards don't have to run the exact > >>> same FW/bootloader binary, so the relevant DTB could be programmed onto > >>> each board. > >>> > >> > >> That has not been the case in any kind of board I’ve worked with. > >> > >> A special firmware image that requires a different programming step at > >> the factory to select the correct DTB for each is always one more thing > >> that can go wrong. > >> > > > > I agree. We have boards with several display modules, even if it seems > > quite easy to know which dtb has to be loaded since we use a prefix > > describing the display module (_pda4, _pda7, etc.) it is a pain for > > customers. Moreover you can add the revision of the board, we have a > > mother board and a cpu module so it can quickly lead to something like > > this: > > at91-sama5d31ek_mb-revc_cm-revd_pda7. > > > > It is only an example, at the moment it is a bit less complicated but I > > am not so far from the reality: sama5d31ek_revc_pda7.dts, > > sama5d33ek_revc_pda4.dts, etc. For a SoC family, we have 27 DTS files… > > > > I bet it’s easy to screw up no? Any horror stories from the factory floor? :) Yes it is. I keep these horror stories for Halloween! At the moment no incident because we are trying hard to find workarounds with the help of our In-system programmer or U-boot but the goal is to not rely on a particular component to do this job. > > > As for the single zImage, we should find a way to have a single DTB. > > > >>>> +While this may in theory work, in practice it is very cumbersome > >>>> +for the following reasons: > >>>> + > >>>> +1. The act of selecting a different boot device tree blob requires > >>>> +a reasonably advanced bootloader with some kind of configuration or > >>>> +scripting capabilities. Sadly this is not the case many times, the > >>>> +bootloader is extremely dumb and can only use a single dt blob. > >>> > >>> You can have several bootloader builds, or even a single build with > >>> something like appended DTB to get an appropriate DTB if the same binary > >>> will otherwise work across all variants of a board. > >>> > >> > >> No, the same DTB will not work across all the variants of a board. > >> > >>> So it's not necessarily true that you need a complex bootloader. > >>> > >> > >>>> +2. On many instances boot time is extremely critical; in some cases > >>>> +there are hard requirements like having working video feeds in under > >>>> +2 seconds from power-up. This leaves an extremely small time budget for > >>>> +boot-up, as low as 500ms to kernel entry. The sanest way to get there > >>>> +is by removing the standard bootloader from the normal boot sequence > >>>> +altogether by having a very small boot shim that loads the kernel and > >>>> +immediately jumps to kernel, like falcon-boot mode in u-boot does. > >>> > >>> Given my previous comments above I don't see why this is relevant. > >>> You're already passing _some_ DTB here, so if you can organise for the > >>> board to statically provide a sane DTB that's fine, or you can resort to > >>> appended DTB if it's not possible to update the board configuration. > >>> > >> > >> You’re missing the point. I can’t use the same DTB for each revision of the > >> board. Each board is similar but it’s not identical. > >> > >>>> +3. Having different DTBs/DTSs for different board revisions easily leads to > >>>> +drift between versions. Since no developer is expected to have every single > >>>> +board revision at hand, things are easy to get out of sync, with board versions > >>>> +failing to boot even though the kernel is up to date. > >>> > >>> I'm not sure I follow. Surely if you don't have every board revision at > >>> hand you can't test quirks exhaustively either? > >>> > >> > >> It’s one less thing to worry about. For example in the current mainline kernel > >> already there is a drift between the beaglebone white and the beaglebone black. > >> > >> Having the same DTS is just easier to keep things in sync. > >> > >>> Additionally you face the problem that two boards of the same variant > >>> could have different base DTBs that you would need to test that each > >>> board's quirks worked for a range of base DTBs. > >>> > >> > >> This is not a valid case. This patch is about boards that have the same base DTB. > >> > >>>> +4. One final problem is the static way that device tree works. > >>>> +For example it might be desirable for various boards to have a way to > >>>> +selectively configure the boot device tree, possibly by the use of command > >>>> +line options. For instance a device might be disabled if a given command line > >>>> +option is present, different configuration to various devices for debugging > >>>> +purposes can be selected and so on. Currently the only way to do so is by > >>>> +recompiling the DTS and installing, which is an chore for developers and > >>>> +a completely unreasonable expectation from end-users. > >>> > >>> I'm not sure I follow here. > >>> > >>> Which devices do you envisage this being the case for? > >>> > >>> Outside of debug scenarios when would you envisage we do this? > >>> > >> > >> We already have to do this on the beaglebone black. The onboard EMMC and HDMI > >> devices conflict with any capes that use the same pins. So you have to > >> have a way to disable them so that the attached cape will work. > >> > >>> For the debug case it seems reasonable to have command line parameters > >>> to get the kernel to do what we want. Just because there's a device in > >>> the DTB that's useful in a debug scenario doesn't mean we have to use it > >>> by default. > >> > >> I don’t follow. Users need this functionality to work. I.e. pass a command > >> line option to use different OPPs etc. Real world usage is messy. > >> > >>> > >>>> +Device Tree quirks solve all those problems by having an in-kernel interface > >>>> +which per-board/platform method can use to selectively modify the device tree > >>>> +right after unflattening. > >>>> + > >>>> +A DT quirk is a subtree of the boot DT that can be applied to > >>>> +a target in the base DT resulting in a modification of the live > >>>> +tree. The format of the quirk nodes is that of a device tree overlay. > >>>> + > >>>> +As an example the following DTS contains a quirk. > >>>> + > >>>> +/ { > >>>> + foo: foo-node { > >>>> + bar = <10>; > >>>> + }; > >>>> + > >>>> + select-quirk = <&quirk>; > >>>> + > >>>> + quirk: quirk { > >>>> + fragment@0 { > >>>> + target = <&foo>; > >>>> + __overlay { > >>>> + bar = <0xf00>; > >>>> + baz = <11>; > >>>> + }; > >>>> + }; > >>>> + }; > >>>> +}; > >>>> + > >>>> +The quirk when applied would result at the following tree: > >>>> + > >>>> +/ { > >>>> + foo: foo-node { > >>>> + bar = <0xf00>; > >>>> + baz = <11>; > >>>> + }; > >>>> + > >>>> + select-quirk = <&quirk>; > >>>> + > >>>> + quirk: quirk { > >>>> + fragment@0 { > >>>> + target = <&foo>; > >>>> + __overlay { > >>>> + bar = <0xf00>; > >>>> + baz = <11>; > >>>> + }; > >>>> + }; > >>>> + }; > >>>> + > >>>> +}; > >>>> + > >>>> +The two public method used to accomplish this are of_quirk_apply_by_node() > >>>> +and of_quirk_apply_by_phandle(); > >>>> + > >>>> +To apply the quirk, a per-platform method can retrieve the phandle from the > >>>> +select-quirk property and pass it to the of_quirk_apply_by_phandle() node. > >>>> + > >>>> +The method which the per-platform method is using to select the quirk to apply > >>>> +is out of the scope of the DT quirk definition, but possible methods include > >>>> +and are not limited to: revision encoding in a GPIO input range, board id > >>>> +located in external or internal EEPROM or flash, DMI board ids, etc. > >>> > >>> It seems rather unfortunate that to get a useful device tree we have to > >>> resort to board-specific mechanisms. That means yet more platform code, > >>> which is rather unfortunate. This would also require any other DT users > >>> to understand and potentially have to sanitize any quirks (e.g. in the > >>> case of Xen). > >> > >> The original internal version of the patches used early platform devices and > >> a generic firmware quirk mechanism, but I was directed to the per-platform > >> method instead. It is perfectly doable to go back at the original implementation > >> but I need to get the ball rolling with a discussion about the internals. > > > > I also think we should used early platform devices to not add platform > > specific code. What were the cons to swith to per-platform method? > > > > Supposedly easier to accept. /me shrugs > > >> > >>> > >>> Mark. > >> > >> Regards > >> > >> — Pantelis > >> > > > > Regards > > > > Ludovic > > Regards > > — Pantelis > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/