[PATCH 2/4] of: DT quirks infrastructure

[Ludovic Desroches]

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 at gmail.com> wrote:
> >>
> >> On 2/19/2015 9:00 AM, Pantelis Antoniou wrote:
> >>> Hi Frank,
> >>>
> >>>> On Feb 19, 2015, at 18:48 , Frank Rowand <frowand.list at gmail.com> wrote:
> >>>>
> >>>> On 2/19/2015 6:29 AM, Pantelis Antoniou wrote:
> >>>>> Hi Mark,
> >>>>>
> >>>>>> On Feb 18, 2015, at 19:31 , Mark Rutland <mark.rutland at arm.com> 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