On Fri, Feb 20, 2015 at 09:21:38AM -0500, Peter Hurley 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. > As you said, we can imagine many reasons to have a failure during the production, having several DTB files will increase the risk. > 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. > > Regards, > Peter Hurley -- 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
