[PATCH 2/4] of: DT quirks infrastructure

[Pantelis Antoniou]

Hi Peter,

> On Feb 20, 2015, at 17:00 , Peter Hurley <peter at hurleysoftware.com> wrote:
> 
> On 02/20/2015 09:35 AM, Ludovic Desroches wrote:
>> 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.
> 
> It's interesting that you don't see the added complexity of open-coding
> the i2c driver or mixing DTS fragments for different designs as increased risk
> (for us all).
> 
> 

You don’t have to use it. Some people really do though. As for increased risk
I expect to see arguments instead of a statement.

>>> 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
> 

Regards

— Pantelis