Re: [PATCH V3 5/7] serial: earlycon: Set UPIO_MEM32BE based on DT properties

[Peter Hurley <peter@hurleysoftware.com>]

Hi Grant,

On 03/27/2015 09:36 PM, Grant Likely wrote:
> On Sun, 01 Mar 2015 17:23:11 -0500
> , Peter Hurley <peter@hurleysoftware.com>
>  wrote:
>> Hi Kevin,
>>
>> On 11/24/2014 06:36 PM, Kevin Cernekee wrote:
>>> If an earlycon (stdout-path) node is being used, check for "big-endian"
>>> or "native-endian" properties and pass the appropriate iotype to the
>>> driver.
>>>
>>> Note that LE sets UPIO_MEM (8-bit) but BE sets UPIO_MEM32BE (32-bit).  The
>>> big-endian property only really makes sense in the context of 32-bit
>>> registers, since 8-bit accesses never require data swapping.
>>>
>>> At some point, the of_earlycon code may want to pass in the reg-io-width,
>>> reg-offset, and reg-shift parameters too.
>>>
>>> Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
>>> ---
>>>  drivers/of/fdt.c              | 7 ++++++-
>>>  drivers/tty/serial/earlycon.c | 4 ++--
>>>  include/linux/serial_core.h   | 2 +-
>>>  3 files changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>>> index 658656f..9d21472 100644
>>> --- a/drivers/of/fdt.c
>>> +++ b/drivers/of/fdt.c
>>> @@ -794,6 +794,8 @@ int __init early_init_dt_scan_chosen_serial(void)
>>>  
>>>  	while (match->compatible[0]) {
>>>  		unsigned long addr;
>>> +		unsigned char iotype = UPIO_MEM;
>>> +
>>>  		if (fdt_node_check_compatible(fdt, offset, match->compatible)) {
>>>  			match++;
>>>  			continue;
>>> @@ -803,7 +805,10 @@ int __init early_init_dt_scan_chosen_serial(void)
>>>  		if (!addr)
>>>  			return -ENXIO;
>>>  
>>> -		of_setup_earlycon(addr, match->data);
>>> +		if (of_fdt_is_big_endian(fdt, offset))
>>> +			iotype = UPIO_MEM32BE;
>>> +
>>> +		of_setup_earlycon(addr, iotype, match->data);
>>
>> I know these got ACKs already but as you point out in the commit log,
>> earlycon _will_ need reg-io-width, reg-offset and reg-shift. Since the
>> distinction between early_init_dt_scan_chosen_serial() and
>> of_setup_earlycon() is arbitrary, I'd rather see of_setup_earlycon()
>> taught to properly decode of_serial driver bindings instead of a
>> stack of parameters to of_setup_earlycon().
>>
>> In fact, this patch allows a mis-defined devicetree to bring up a
>> functioning earlycon because the 'big-endian' property is directly
>> associated with UPIO_MEM32BE, which will create incompatibility problems
>> when DT earlycon is fixed to decode the of_serial DT bindings.
> 
> That's a good point. This hasn't been merged yet, so there isn't any
> impact on addressing this. I would propose that for consistency, the
> earlycon code should always default to 8-bit access. if big-endian
> accesses are required, then reg-io-width + big-endian must be specified.
> 
> Something like the following would do it and would be future-proof. We
> can add support for 16 or 64bit big or little endian access if it ever
> became necessary.

I was planning on adding MEM32BE support to OF earlycon on top of my
patch series 'OF earlycon cleanup', which adds full support for the
of_serial driver DT properties (among other things).

Unfortunately, that series is waiting on two things:
1. libfdt upstream patch, which I submitted but was referred back to me
to add test cases. That was 3 weeks ago and I simply haven't had a free
day to burn to figure out how their test matrix is organized. I don't
think that's going to change anytime soon; I might just abandon that patch
and do the string manipulation on the stack.

ATM, earlycon is still broken if stdout-path options have been set.

2. Rob never got back to me on my query [1] to unify the OF_EARLYCON_DECLARE
macro with the EARLYCON_DECLARE macro so that all earlycon consoles
are named.

Regards,
Peter Hurley

[1] https://lkml.org/lkml/2015/3/6/571