Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754605AbbBTOiX (ORCPT ); Fri, 20 Feb 2015 09:38:23 -0500 Received: from mail-we0-f169.google.com ([74.125.82.169]:41013 "EHLO mail-we0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754297AbbBTOiV convert rfc822-to-8bit (ORCPT ); Fri, 20 Feb 2015 09:38:21 -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: <54E742F2.80506@hurleysoftware.com> Date: Fri, 20 Feb 2015 16:38:14 +0200 Cc: frowand.list@gmail.com, Mark Rutland , "devicetree@vger.kernel.org" , Tony Lindgren , Koen Kooi , Nicolas Ferre , "linux-kernel@vger.kernel.org" , Grant Likely , Ludovic Desroches , "linux-arm-kernel@lists.infradead.org" , Matt Porter , Guenter Roeck Content-Transfer-Encoding: 8BIT Message-Id: <3EEA42F7-393E-46AD-B732-BCA7D6E068CE@konsulko.com> References: <1424271576-1952-1-git-send-email-pantelis.antoniou@konsulko.com> <1424271576-1952-3-git-send-email-pantelis.antoniou@konsulko.com> <20150218154106.GC29429@leverpostej> <20150218173115.GG29429@leverpostej> <76BD1B22-BAED-4205-9B34-186907CE0217@konsulko.com> <54E613E7.2020405@gmail.com> <670D0881-DBF0-45E8-A502-A6DB2B77A750@konsulko.com> <54E61DD2.3060002@gmail.com> <53F2F94C-0C43-4A54-B8CD-EEC454A0AC19@konsulko.com> <54E742F2.80506@hurleysoftware.com> To: Peter Hurley 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: 8072 Lines: 177 Hi Peter, > On Feb 20, 2015, at 16:21 , Peter Hurley wrote: > > On 02/19/2015 12:38 PM, Pantelis Antoniou wrote: >> >>> On Feb 19, 2015, at 19:30 , Frank Rowand wrote: >>> >>> On 2/19/2015 9:00 AM, Pantelis Antoniou wrote: >>>> Hi Frank, >>>> >>>>> On Feb 19, 2015, at 18:48 , Frank Rowand wrote: >>>>> >>>>> On 2/19/2015 6:29 AM, Pantelis Antoniou wrote: >>>>>> Hi Mark, >>>>>> >>>>>>> On Feb 18, 2015, at 19:31 , Mark Rutland wrote: >>>>>>> >>>>>>>>>> +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. >>>>>>> >>>>>>> I wasn't on about the DTB. I was on about the loader binary, in the case >>>>>>> the FW/bootloader could be common even if the DTB couldn't. >>>>>>> >>>>>>> To some extent there must be a DTB that will work across all variants >>>>>>> (albeit with limited utility) or the quirk approach wouldn't work… >>>>>>> >>>>>> >>>>>> That’s not correct; the only part of the DTB that needs to be common >>>>>> is the model property that would allow the quirk detection logic to fire. >>>>>> >>>>>> So, there is a base DTB that will work on all variants, but that only means >>>>>> that it will work only up to the point that the quirk detector method >>>>>> can work. So while in recommended practice there are common subsets >>>>>> of the DTB that might work, they might be unsafe. >>>>>> >>>>>> For instance on the beaglebone the regulator configuration is different >>>>>> between white and black, it is imperative you get them right otherwise >>>>>> you risk board damage. >>>>>> >>>>>>>>> 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. >>>>>>> >>>>>>> I think you've misunderstood my point. If you program the board with the >>>>>>> relevant DTB, or use appended DTB, then you will pass the correct DTB to >>>>>>> the kernel without need for quirks. >>>>>>> >>>>>>> I understand that each variant is somewhat incompatible (and hence needs >>>>>>> its own DTB). >>>>>> >>>>>> In theory it might work, in practice this does not. Ludovic mentioned that they >>>>>> have 27 different DTBs in use at the moment. At a relatively common 60k per DTB >>>>>> that’s 27x60k = 1.6MB of DTBs, that need to be installed. >>>>> >>>>> < snip > >>>>> >>>>> Or you can install the correct DTB on the board. You trust your manufacturing line >>>>> to install the correct resistors. You trust your manufacturing line to install the >>>>> correct kernel version (eg an updated version to resolve a security issue). >>>>> >>>>> I thought the DT blob was supposed to follow the same standard that other OS's or >>>>> bootloaders understood. Are you willing to break that? (This is one of those >>>>> ripples I mentioned in my other emails.) >>>>> >>>> >>>> Trust no-one. >>>> >>>> This is one of those things that the kernel community doesn’t understand which makes people >>>> who push product quite mad. >>>> >>>> Engineering a product is not only about meeting customer spec, in order to turn a profit >>>> the whole endeavor must be engineered as well for manufacturability. >>>> >>>> Yes, you can always manually install files in the bootloader. For 1 board no problem. >>>> For 10 doable. For 100 I guess you can hire an extra guy. For 1 million? Guess what, >>>> instead of turning a profit you’re losing money if you only have a few cents of profit >>>> per unit. >>> >>> I'm not installing physical components manually. Why would I be installing software >>> manually? (rhetorical question) >>> >> >> Because on high volume product runs the flash comes preprogrammed and is soldered as is. >> >> Having a single binary to flash to every revision of the board makes logistics considerably >> easier. >> >> Having to boot and tweak the bootloader settings to select the correct dtb (even if it’s present >> on the flash medium) takes time and is error-prone. >> >> Factory time == money, errors == money. >> >>>> >>>> No knobs to tweak means no knobs to break. And a broken knob can have pretty bad consequences >>>> for a few million units. >>> >>> And you produce a few million units before testing that the first one off the line works? >>> >> >> The first one off the line works. The rest will get some burn in and functional testing if you’re >> lucky. In many cases where the product is very cheap it might make financial sense to just ship >> as is and deal with recalls, if you’re reasonably happy after a little bit of statistical sampling. >> >> Hardware is hard :) > > I'm failing to see how this series improves your manufacturing process at all. > > 1. Won't you have to provide the factory with different eeprom images for the > White and Black? You _trust_ them to get that right, or more likely, you > have process control procedures in place so that you don't get 1 million Blacks > flashed with the White eeprom image. > > 2. The White and Black use different memory technology so it's not as if the > eMMC from the Black will end up on the White SMT line (or vice versa). > > 3 For that matter, why wouldn't you worry that all the microSD cards intended > for the White were accidentally assembled with the first 50,000 Blacks; at > that point you're losing a lot more than a few cents of profit. And that has > nothing to do with what image you provided. > > 3. The factory is just as likely to use some other customer's image by accident, > so you're just as likely to have the same failure rate if you have no test > process at the factory. > > 4. If you're using offline programming, the image has to be tested after > reflow anyway. > > IOW, your QA process will not change at all == same cost. > I never said this fixes every case where someone might screw up, I just said that it makes it extremely less likely. And no-one is holding the beaglebone as the paragon of good design process for ease of manufacturing as far as I know. > Regards, > Peter Hurley 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/