Hi Peter, > On Feb 20, 2015, at 16:21 , Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: > > On 02/19/2015 12:38 PM, Pantelis Antoniou wrote: >> >>> On Feb 19, 2015, at 19:30 , Frank Rowand <frowand.list@xxxxxxxxx> wrote: >>> >>> On 2/19/2015 9:00 AM, Pantelis Antoniou wrote: >>>> Hi Frank, >>>> >>>>> On Feb 19, 2015, at 18:48 , Frank Rowand <frowand.list@xxxxxxxxx> wrote: >>>>> >>>>> On 2/19/2015 6:29 AM, Pantelis Antoniou wrote: >>>>>> Hi Mark, >>>>>> >>>>>>> On Feb 18, 2015, at 19:31 , Mark Rutland <mark.rutland@xxxxxxx> 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 devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
