Re: [PATCH v1 3/4] xen/arm: Add SCIFA UART support for early printk

From: Julien Grall <julien.grall@arm.com>
To: Oleksandr Tyshchenko <olekstysh@gmail.com>
Cc: Artem Mygaiev <Artem_Mygaiev@epam.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Subject: Re: [PATCH v1 3/4] xen/arm: Add SCIFA UART support for early printk
Date: Tue, 7 Aug 2018 16:22:08 +0100	[thread overview]
Message-ID: <6ff865a9-0f16-1139-3ee4-5674627cfde1@arm.com> (raw)
In-Reply-To: <CAPD2p-nPrrXJ-4Vu+6WhHa+FbhTa5vfM91-e2Pi2XPZR+852ig@mail.gmail.com>

On 07/08/18 15:28, Oleksandr Tyshchenko wrote:
> On Tue, Aug 7, 2018 at 4:48 PM, Julien Grall <julien.grall@arm.com> wrote:
>> Hi,
> 
> Hi, Julien
> 
>>
>> On 06/08/18 19:35, Oleksandr Tyshchenko wrote:
>>>
>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>
>>> Add support for Renesas "Stout" development board based on
>>> R-Car H2 SoC which has SCIFA compatible UART.
>>>
>>> Actually existing SCIF UART support (debug-scif.inc) and
>>> newly added SCIFA UART support (debug-scifa.inc) differ only
>>> in registers offsets.
>>
>> In that case, could we just extend debug-scif.inc?
> I was thinking about that, but couldn't find suitable solution without
> adding extra config option.

I had the config option in my mind. But it may not be worth it.

> As I understand, we need to recognize in run-time somehow which
> interface is present to use proper register offsets,
> so in UART driver it is easy to recognize using device-tree compatible
> string, but what to do here in such an early code.
> 
>>
>>>
>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>> CC: Stefano Stabellini <sstabellini@kernel.org>
>>> CC: Julien Grall <julien.grall@arm.com>
>>> ---
>>>    docs/misc/arm/early-printk.txt     |  3 ++-
>>>    xen/arch/arm/Rules.mk              |  1 +
>>>    xen/arch/arm/arm32/debug-scifa.inc | 51
>>> ++++++++++++++++++++++++++++++++++++++
>>>    3 files changed, 54 insertions(+), 1 deletion(-)
>>>    create mode 100644 xen/arch/arm/arm32/debug-scifa.inc
>>>
>>> diff --git a/docs/misc/arm/early-printk.txt
>>> b/docs/misc/arm/early-printk.txt
>>> index f765f59..f1b55d3 100644
>>> --- a/docs/misc/arm/early-printk.txt
>>> +++ b/docs/misc/arm/early-printk.txt
>>> @@ -39,12 +39,13 @@ the name of the machine:
>>>      - fastmodel: printk on ARM Fastmodel software emulators
>>>      - hikey960: printk with pl011 with Hikey 960
>>>      - juno: printk with pl011 on Juno platform
>>> -  - lager: printk with SCIF0 on Renesas R-Car H2 processors
>>> +  - lager: printk with SCIF0 on Renesas Lager board (R-Car H2 processor)
>>
>>
>> Why this change?
> This sentence was not entirely correct. Since SCIF0 interface is
> applicable for Lager
> board, but is not applicable for Stout board which also based on R-Car
> H2 processor.
> Shall I create a separate patch for this small correction?

Yes please.

> 
>>
>>>      - midway: printk with the pl011 on Calxeda Midway processors
>>>      - mvebu: printk with the MVEBU for Marvell Armada 3700 SoCs
>>>      - omap5432: printk with UART3 on TI OMAP5432 processors
>>>      - rcar3: printk with SCIF2 on Renesas R-Car Gen3 processors
>>>      - seattle: printk with pl011 for AMD Seattle processor
>>> +  - stout: printk with SCIFA0 on Renesas Stout board (R-Car H2 processor)
>>
>>
>> I have started to look at porting that to Kconfig ealyprintk and it is a
>> massive pain. So I would tend to prefer if we avoid adding more convenience
>> alias and instead document on the wiki page how to use earlyprintk for that.
> I will update a wiki page. Shall I drop this string in early-printk.txt?

Yes please.

Cheers,

-- 
Julien Grall

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