Re: [PATCH 2/4] of: DT quirks infrastructure

[Ludovic Desroches <ludovic.desroches@atmel.com>]

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@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@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)

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
>