Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752720AbbBRQc7 (ORCPT ); Wed, 18 Feb 2015 11:32:59 -0500 Received: from eusmtp01.atmel.com ([212.144.249.242]:33258 "EHLO eusmtp01.atmel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752492AbbBRQcz (ORCPT ); Wed, 18 Feb 2015 11:32:55 -0500 Date: Wed, 18 Feb 2015 17:32:38 +0100 From: Ludovic Desroches To: Pantelis Antoniou CC: Mark Rutland , Grant Likely , Matt Porter , Koen Kooi , Guenter Roeck , Ludovic Desroches , 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: <20150218163238.GG32600@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> 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: 10373 Lines: 248 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... 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? > > > > > Mark. > > Regards > > — Pantelis > Regards Ludovic -- 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/