[PATCH 2/4] of: DT quirks infrastructure

[Pantelis Antoniou]

Hi Peter,

> On Feb 20, 2015, at 16:21 , Peter Hurley <peter at hurleysoftware.com> 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.
> 
> 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.
> 

I never said this fixes every case where someone might screw up, I just said
that it makes it extremely less likely.

And no-one is holding the beaglebone as the paragon of good design process for
ease of manufacturing as far as I know.
 
> Regards,
> Peter Hurley

Regards

— Pantelis