Re: [PATCH 1/2] printk: export pr_flush()

[Esben Haabendal <esben@geanix.com>]

John Ogness <john.ogness@linutronix.de> writes:

> On 2024-04-02, Esben Haabendal <esben@geanix.com> wrote:
>> Are there any other examples other than 8250 for how to port a driver to
>> nbcon?
>
> No. 8250 is the first example. And even this example is changing a bit
> with each new RT version as the underlying APIs and semantics have been
> changing.
>
>> What is the plans for porting all of this to mainline?  Should all
>> drivers be ported first, or will a solution to prevent this type of
>> regression be implemented at some later time?
>
> First off, I want to mention that this behavior is only with the
> PREEMPT_RT preemption model. And it has been this way for a long
> time. The behavior of legacy consoles for other preemption models is the
> same as mainline now. So from a mainline perspective, I would not
> consider it a regression.

Got it. I think the last version where I know that this problem was not
seen with PREEMPT_RT was 4.19. And yes, that is a long time ago :)

> It is hard to say how this will actually play out though. We are
> bringing the printk/console rework to mainline piece by piece. Until now
> every piece has needed significant modifications in order to be accepted
> mainline. (This is the main reason the 8250 driver keeps needing
> changes.)
>
> The updated 8250 driver will be the last piece to go mainline. Only then
> would all the APIs, semantics, and documentation be official. Hopefully
> it is quite similar to what we have in the PREEMPT_RT tree now.
>
> I hope that once the printk/console rework is complete, there will be a
> big push to port over the other serial drivers. Most of them are just
> copy/paste of each other, which should simplify the porting. And since
> the new nbcon consoles provide real advantages, there will also be
> incentive to take on the porting effort.
>
> Finally, if someone wanted to try porting another driver for PREEMPT_RT
> (for example, the imx serial console), I would certainly be interested
> in reviewing and integrating the patches for the PREEMPT_RT tree.

Sounds good. I have looked at porting imx uart driver to nbcon, and have
something working now. It does smell quite a lot of copy-paste
behaviour, but I will send it as an RFC for you to see when I have
tested it a bit more.

The write_thread and write_atomic functions share quite a lot of code,
carrying half of the copy-paste smell.
The other half is the inner loop of the write_thread function, which is
almost identical in 8250 and imx drivers.

I guess there is room to refactor this to avoid this amount of
copy-paste before starting mass-porting.

/Esben