So to use a MMIO based 8250 as "earlycon", either we need to have some
sort of fixmap
(as in case of x86) or otherwise we need to have a ioremap_nocache()
version that works
even before kmem_cache_init() is called.
I am not sure how many architectures will have such form of ioremap
other than maybe
nommu variants.
Greg,
Has anyone used the ioremap path so far ? If not, then why have it
there as usual ioremap
can be used only after mm_init(), but this driver whenever used will
get called earlier than that ?
Regards,
Sukanto
On Sun, Jan 29, 2012 at 3:03 PM, Dave Hylands <dhylands@xxxxxxxxx> wrote:
> Hi Sukanto,
>
> On Sat, Jan 28, 2012 at 11:25 PM, Sukanto Ghosh
> <sukanto.cse.iitb@xxxxxxxxx> wrote:
>> Hi Dave,
>>
>> If you look into start_kernel() the call to parse_early_param()
>> precedes mm_init().
>> parse_early_param() eventually calls do_early_param().
>> do_early_param() parses for "earlycon" in kernel commandline and then calls the
>> setup_function associated with earlycon.
>>
>> I have in my commandline: earlycon=uart8250,mmio32,0x10000000,9600
>>
>> Call flow starting from drivers/tty/serial/8250_early.c will lead to kmalloc
>>
>> early_param("early_con", setup_early_serial8250_console)
>> setup_early_serial8250_console()
>> early_serial8250_setup()
>> parse_options()
>> ioremap_nocache()
>> ... arch-specific-ioremap
>> -- some form of arch specific __ioremap_caller
>> --- get_vm_area_caller()
>> __get_vm_area_node
>> kzalloc_node
>> kmalloc_node
>> kmalloc
>
> It looks like CONFIG_FIX_EARLYCON_MEM defaults to y (on x86 anyways),
> so this would cause the path that doesn't call ioremam_nocache to be
> taken.
>
> I'm guessing that if you specify earycon, then you also need to ensure
> that CONFIG_FIX_EARLYCON is set.
>
> You didn't mention which architecture you were using. ARM has an
> early_printk which is sort of like an early console.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
--
Regards,
Sukanto Ghosh
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