linux-kernel - Re: [V2, 2/6] tty: serial: lpuart: add little endian 32 bit register support

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Date:   Tue, 23 May 2017 08:24:18 +0300
From:   Nikita Yushchenko <nikita.yoush@...entembedded.com>
To:     Dong Aisheng <dongas86@...il.com>
Cc:     "A.S. Dong" <aisheng.dong@....com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Andy Duan <fugang.duan@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "Y.B. Lu" <yangbo.lu@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stefan@...er.ch" <stefan@...er.ch>,
        Mingkai Hu <mingkai.hu@....com>,
        "jslaby@...e.com" <jslaby@...e.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [V2, 2/6] tty: serial: lpuart: add little endian 32 bit register
 support

Hi,

>>>> Alternative solution could be - have separate write path for earlycon.
>>>
>>> It looks to me having the same issue with a separate write patch
>>> for earlycon as we still need distinguish Little or Big endian
>>> for Layerscape and IMX.
>>>
>>>> At a glance, it is dozen lines of code.
>>>
>>> Would you please show some sample code?
>>
>> Do not reuse lpuart32_console_putchar() in earlycon code.
>>
>> Have two sets of early_setup/early_write/putchar - for BE and
>> defaut-endian earlycon. And in these putchar's do not use
>> lpuart_(read|write).
>>
> 
> Isn't that introducing another consistency break after fix one
> consistency break?
> 
> If doing that, we then have two register read/write APIs.
> One for normal driver operation by dynamically checking lpuart_is_be
> property to distinguish the endian difference problem.
> Another is specifically implemented for only early console read/write
> and use hardcoded way to read/write register directly instead of using
> the standard API lpuart32_read/write, like follows:
> e.g.
> lpuart32_le_console_write() {
> 	 writel();
> }
> 
> lpuart32_be_console_write() {
> 	 iowrite32be()
> }
> This also makes the driver a bit strange and ugly.
> 
> It looks to me both way are trade offs and the later one seems sacrifice
> more. And i doubt if it's really necessary for probably a no real gain
> purpose as the FPGA you mentioned is a theoretical case and less
> possibility to exist.
> 
> I'm still wondering how about keep using the exist way and adding more
> information in code to explain why use a global var?

I've checked other driver under drivers/tty/serial/, for examples of
similar cases.

Please look at serial8250_early_in() / serial8250_early_out() ?
These do handle different endian, via port->iotype

Another example is drivers/tty/serial/samsung.c, where
port->private_data is initialized and used.

Nikita