On Thu, Feb 19, 2015 at 09:30:58AM -0800, Frank Rowand 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) It is not only about manufacturing. You can provide software updates and trust me even if it seems easy to identify which dtb you have to load with a good naming, some customers will use the bad one. Other use case, we have a cpu module with the nand flash and a mother board, we put the cpu module on another mother board with a different revision, you don't have to update your dtb. Maybe it is not necessary but it is a ease of use. I don't understand how we could want a single zImage (even if it helps to clean the code) and we don't take care about dtb stuff. > > > > > 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? > > > > > And frankly I don’t care what other OSes do. If you were to take a look at the sorry DT support > > they have you’d be amazed. > > > > I would be very surprised if there’s another OS out there that can boot with a late Linux DTB. > > > >> -Frank > > > > Regards > > > > — Pantelis > > > > PS. For a real use case please take a look at the answer Guenter gave on this thread a little > > while back. > > > > My previous comments were written after reading Guenter's comment. > > -Frank > -- 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
