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) > > 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
