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

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Message-ID: <20170531080742.GD16812@b29396-OptiPlex-7040>
Date:   Wed, 31 May 2017 16:07:42 +0800
From:   Dong Aisheng <dongas86@...il.com>
To:     Nikita Yushchenko <nikita.yoush@...entembedded.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 Nikita,

On Tue, May 23, 2017 at 08:24:18AM +0300, Nikita Yushchenko wrote:
> 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
> 

Well, that does inspire me.

With struct uart_port's iotype member, the global lpuart_is_be can be
gone. We can update lpuart32_read/write API to take the reference of
of structure uart_port, then the API knows the endian information and
can use the correct further IO accessor accordingly.

And most importantly, it also works with earlycon.

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

If using iotype, seems no need private_data anymore.
Will try and send the new series later with you CCed to help review.

Thanks for the advice.

Regards
Dong Aisheng