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

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Date:   Wed, 17 May 2017 14:09:57 +0800
From:   Dong Aisheng <dongas86@...il.com>
To:     Nikita Yushchenko <nikita.yoush@...entembedded.com>
Cc:     Dong Aisheng <aisheng.dong@....com>, linux-serial@...r.kernel.org,
        fugang.duan@....com, gregkh@...uxfoundation.org, yangbo.lu@....com,
        linux-kernel@...r.kernel.org, stefan@...er.ch, Mingkai.Hu@....com,
        jslaby@...e.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [V2, 2/6] tty: serial: lpuart: add little endian 32 bit register
 support

On Wed, May 17, 2017 at 08:50:39AM +0300, Nikita Yushchenko wrote:
> >>>>>  static u32 lpuart32_read(void __iomem *addr)
> >>>>>  {
> >>>>> -	return ioread32be(addr);
> >>>>> +	return lpuart_is_be ? ioread32be(addr) : readl(addr);
> >>>>>  }
> >>>>>  
> >>>>>  static void lpuart32_write(u32 val, void __iomem *addr)
> >>>>>  {
> >>>>> -	iowrite32be(val, addr);
> >>>>> +	if (lpuart_is_be)
> >>>>> +		iowrite32be(val, addr);
> >>>>> +	else
> >>>>> +		writel(val, addr);
> >>>>>  }
> >>>>
> >>>> What if this is ever executed on big endian system?
> >>>>
> >>>
> >>> Sorry, not catching the point...
> >>>
> >>> What issues will meet?
> >>
> >> Isn't writel() in host endian?
> > 
> > On big endian systems, it is supposed to run iowrite32be.
> 
> Your code states, "force BE if lpuart_is_be, don't care otherwise".
> This semantics looks questionable for code reviewer.
> If driver handles endian, should't it be explicit in both cases?
> And if indeed driver means handling BE explicitly, but don't caring
> otherwise, maybe variable name should suggest that (i.e. "force_be")?
> 

For lpuart32, only two cases that LS platforms is Big endian while
IMX is little endian.

It's SoC IP native property, i don't think force_be is better.

Regards
Dong Aisheng

> Although driver maintainer could think differently. I won't insist on this.
> 
> Nikita