linux-kernel - Re: [PATCH v3] serial: 8250_uniphier: add UniPhier serial driver

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Date:	Tue, 19 May 2015 13:31:21 +0200
From:	Matthias Brugger <matthias.bgg@...il.com>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Peter Hurley <peter@...leysoftware.com>,
	John Crispin <blogic@...nwrt.org>,
	Tony Lindgren <tony@...mide.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joachim Eastwood <manabian@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org,
	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	<"linux-arm-kernel@...ts.infradead.org\" <linux-arm-kernel@...ts.infradead.org>"@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH v3] serial: 8250_uniphier: add UniPhier serial driver

2015-05-19 13:18 GMT+02:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
> Hi Alan,
>
>
> 2015-05-19 18:48 GMT+09:00 One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>:
>>> I intentionally use deeper indentation for *_SHIFT
>>> because I want to clearly show  UNIPHIER_UART_LCR_SHIFT
>>> belongs to UNIPHIER_UART_LCR_MCR register.
>>
>> Seems sensible to do it that way to me and a lot of other bits of the
>> kernel do.
>>
>> The only other question I have is about the unipher_serial_out. If I am
>> writing a "special" register then the sequence becomes
>>
>> - read 32bits
>> - modify
>> - write 32bits
>>
>> That means that it's no longer atomic safe as the kernel expects.
>> Checking the users FCR seems safe, LCR is probably safe and MCR likewise
>> so I don't see a problem but I think it's worth noting in case anyone
>> else does.
>
> Uh, I missed this.
> I am a bit afraid what if LCR and MCR are updated at the same time.
>
> Is it better to add mutex for writing special case registers?
>
>     if (normal) {
>             writel(value, p->membase + offset);
>     } else {
>             /* special case: two registers share the same address. */
>             u32 tmp = readl(p->membase + offset);
>             struct uniphier8250_priv *priv = p->private_data;
>
>             mutex_lock(&priv->atomic_write_lock);
>             tmp &= ~(0xff << valshift);
>             tmp |= value << valshift;
>             writel(tmp, p->membase + offset);
>             mutex_unlock(&priv->atomic_write_lock);
>     }
>
>
> If it is OK, I can fix it in v4.
>
>
>
>
>> Finally can you add a comment in serial_in and serial_out where one
>> switch case drops through into the next so its obvious to anyone looking
>> at Coverity and other analyser output that this drop through was
>> intentional ?
>
> I thought about it, too.
>
> My previous version was as follows:
>
>
> +#define UNIPHIER_UART_CHAR_FCR 3
> +#define     UNIPHIER_UART_CHAR_SHIFT   8       /* Character Register */
> +#define     UNIPHIER_UART_FCR_SHIFT    0       /* FIFO Control Register */
> +#define UNIPHIER_UART_LCR_MCR  4
> +#define     UNIPHIER_UART_LCR_SHIFT    8       /* Line Control Register */
> +#define     UNIPHIER_UART_MCR_SHIFT    0       /* Modem Control Register */
> +#define UNIPHIER_UART_DLR      9               /* Divisor Latch Register */
>
> [snip]
>
> +static void uniphier_serial_out(struct uart_port *p, int offset, int value)
> +{
> +       int valshift = 0;
> +       bool normal = false;
> +
> +       switch (offset) {
> +       case UART_FCR:
> +               offset = UNIPHIER_UART_CHAR_FCR;
> +               valshift = UNIPHIER_UART_FCR_SHIFT;
> +               break;
> +       case UART_LCR:
> +               offset = UNIPHIER_UART_LCR_MCR;
> +               valshift = UNIPHIER_UART_LCR_SHIFT;
> +               /* Divisor latch access bit does not exist. */
> +               value &= ~(UART_LCR_DLAB << valshift);
> +               break;
> +       case UART_MCR:
> +               offset = UNIPHIER_UART_LCR_MCR;
> +               valshift = UNIPHIER_UART_MCR_SHIFT;
> +               break;
> +       default:
> +               normal = true;
> +               break;
> +       }
>
>
>
> I thought it was clear to anyone although it was a bit redundant and
> Matthias was opposed to it.
>
> I personally prefer clear code to tricky code that requires comments.

Me too :)
What I wanted to say was, that in the case statement you don't need to
set valshift to zero as this is the default value when entering
uniphier_serial_out. This way you can get rid of
UNIPHIER_UART_MCR_SHIFT and UNIPHIER_UART_FCR_SHIFT.

Sorry, I didn't explained myself.

Cheers,
Matthias
-- 
motzblog.wordpress.com
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