[PATCH 2/4] of: DT quirks infrastructure

[Pantelis Antoniou]

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.

No knobs to tweak means no knobs to break. And a broken knob can have pretty bad consequences
for a few million units. 

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.