Re: [PATCH v7 00/22] RISC-V Kendryte K210 support improvements

[Damien Le Moal <Damien.LeMoal@xxxxxxx>]

On Thu, 2020-12-10 at 11:04 +0100, Geert Uytterhoeven wrote:
> Hi Damien,
> 
> On Thu, Dec 10, 2020 at 4:41 AM Damien Le Moal <damien.lemoal@xxxxxxx> wrote:
> > Changes from v6:
> > * Annotate struct platform_driver variables with __refdata to avoid
> >   section mismatch compilation errors
> 
> Blindly following the advice from kernel test robot <lkp@xxxxxxxxx> is
> not always a good idea:
> 
>     The variable k210_rst_driver references
>     the function __init set_reset_devices()
>     If the reference is valid then annotate the
>     variable with or __refdata (see linux/init.h) or name the variable:
> 
> If your driver's probe function is annotated with __init, you cannot
> have a pointer to it in the driver structure, as any binding done after
> the freeing of initmem will cause a crash.  Adding the __refdata merely
> suppresses the warning, and won't avoid the crash.

Hmm... I must be misunderstanding something here. free_initmem() is called from
kernel_init() right before starting the user init process. That is late enough
that all drivers are already probed and initialized. At least that is what I
thought, especially considering that none of the k210 drivers can be modules
and are all builtin. What am I missing here ? 

> There are two solutions for this:
>   1. Remove the .probe pointer, and use platform_driver_probe() instead
>      of platform_driver_register().
>      This guarantees the probe method will be called only once, before
>      initmem is freed, but does mean that probe deferral cannot work.
>   2. Drop the __init annotation.
>      This means the probe method can be called anytime, and supports
>      both probe deferral and driver unbind/rebind cycles.
> 
> Given the limited amount of RAM on k210, I think option 1 is preferred,
> but may require careful tuning of the initialization order using
> *_initcall*(), to make sure probe deferral won't ever be needed.

I really would prefer to avoid the *_initcall() dance. That is not pretty.
Simply removing the __init annotation, I see 766 pages free with "cat
/proc/vmstat" right after boot, and the same number with __init too... So it
does not look like the impact is significant. The biggest probe function is for
the clock driver and this one seems fine with __init since it uses
CLK_OF_DECLARE(). I do have a doubt about this now though. Looking at other
drivers, all seem to have the __init annotation for their clock driver init
function.

So I think I will go with option 2. It is simpler and safer. We can always
revisit and optimize later. I would prefer this series to land first :)

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds

-- 
Damien Le Moal
Western Digital