Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752565AbbBRQjM (ORCPT ); Wed, 18 Feb 2015 11:39:12 -0500 Received: from mail-wg0-f41.google.com ([74.125.82.41]:42502 "EHLO mail-wg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752283AbbBRQjI convert rfc822-to-8bit (ORCPT ); Wed, 18 Feb 2015 11:39:08 -0500 Subject: Re: [PATCH 2/4] of: DT quirks infrastructure Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2070.6\)) Content-Type: text/plain; charset=utf-8 From: Pantelis Antoniou In-Reply-To: <20150218163238.GG32600@odux.rfo.atmel.com> Date: Wed, 18 Feb 2015 18:39:01 +0200 Cc: 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" Content-Transfer-Encoding: 8BIT Message-Id: 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> To: Ludovic Desroches X-Mailer: Apple Mail (2.2070.6) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 10722 Lines: 263 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? :) > 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/