Re: [RFC 6/6] xen/arm: Replace early_printk call to printk call

[Julien Grall <julien.grall@linaro.org>]

On 20/02/14 11:20, Ian Campbell wrote:
> On Thu, 2014-02-20 at 11:14 +0000, Julien Grall wrote:
>>
>> On 20/02/14 11:05, Ian Campbell wrote:
>>> On Thu, 2014-02-20 at 11:01 +0000, Julien Grall wrote:
>>>>
>>>> On 20/02/14 09:04, Ian Campbell wrote:
>>>>> I was actually thinking more along the lines of a .word at a defined
>>>>> offset which you could hex edit to a specific value to activate a
>>>>> particular flavour of early printk handling. That would be sufficient
>>>>> e.g. for osstest to activate the appropriate stuff for the specific
>>>>> platform.
>>>>
>>>> I don't see useful use case to have a such early printk implementation
>>>> in Xen. When the board is fully supported, failed at early stage (e.g
>>>> before console is initialized) is very unlikely. At least if you don't
>>>> play with memory.
>>>
>>> a) there are boards which aren't fully supported, getting some debug out
>>> of a distro package might be useful
>>
>> Few months ago we have decided to allow early printk only when Xen is
>> compiled with debug enabled. It seems a big mistake to ship distro with
>> debug enabled :).
>
> This was because earlyprintk only supports a static single configuration
> at compile time. If that restriction was lifted then there would be no
> reason to limit earlyprintk to debug builds.
>
>>> b) even for boards which are fully supported there may still be bugs
>>> which only appear under particular circumstances.
>>
>> I understand this use case. If I understand your previous mail the
>> solution would me "Hex editing manually the Xen binary to set the early
>> printk", right? If so, you are assuming that the distro (or anything
>> else) is proving the zImage. Otherwise the developper has to:
>>      - unpack from the uImage
>>      - editing the zImage
>>      - recreate the uImage
>
> No distro would ship the actual uImage, it's too machine specific.
>
> I would expect this to be used by running:
> 	xen-enable-early-printk /boot/xen midway
>
> where xen-enable-early-printk is a simple tool we provide.

And a similar one to disable, I guess.

>
> Then if a uIamge is then required then this would be generated by
> whatever distro tooling would have generated it in the non-early-printk
> case, by rerunning that tool.

Sounds good. Do you plan to work on it?

It would be nice to have this item on the ARM todo page.

-- 
Julien Grall