linux-kernel - Re: [PATCH] tty/serial: samsung: Add earlycon support

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Date:	Mon, 22 Sep 2014 01:19:46 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Alim Akhtar <alim.akhtar@...il.com>
CC:	Alim Akhtar <alim.akhtar@...sung.com>, gregkh@...uxfoundation.org,
	Rob Herring <robh@...nel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas P Abraham <thomas.ab@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] tty/serial: samsung: Add earlycon support

On 22.09.2014 01:10, Alim Akhtar wrote:

[snip]

>>> As you said there is no support for ioremap on ARM, so this is not
>>> tested on ARM.
>>
>> Don't forget that this driver is primarily targeted for ARM platforms
>> (versus just one ARM64-based Exynos7), so either this feature should be
>> clearly added as ARM64-specific (and compiled in conditionally) or made
>> work for all supported platforms.
>>
> Well, this will work on every platform which uses
> "samsung,exynos4210-uart" as a UART controller.
> Exynos7 also use same, there is nothing special about ARM64 bit here.
> please see[1].
> For "s3c24xx-uart", this has to be implemented separably as that is a
> bit different.

For this feature, they differ only in locations and lengths of
FIFO-related bit fields. We had this already implemented for all
hardware variants and so my recommendation to reuse that code.

>>>>
>>>>>               smh     Use ARM semihosting calls for early console.
>>>>>
>>>>>       earlyprintk=    [X86,SH,BLACKFIN,ARM,M68k]

[snip]

> Tomasz/Marek,
> Do you think something like below make sense here?
> 
> +static void exynos4210_serial_console_putc(struct uart_port *port, int ch)
> +{
> +       while (!(readl(port->membase + S3C2410_UFCON) & S3C2410_UFCON_FIFOMODE))
> +               ;

Waiting in a loop for a software-writable FIFO mode enable bit doesn't
look reasonable to me. Probably a typo?

> +
> +       wr_regb(port, S3C2410_UTXH, ch);
> +
> +       while ((readl(port->membase + S3C2410_UFSTAT) & S5PV210_UFSTAT_TXFULL))
> +               ;

I wonder if you need to wait for the character to be sent. If you ensure
before writing a character that there is no previous one in the buffer
or there is space in FIFO then I believe you should be fine.

> +}
> 
> and call exynos4210_serial_console_putc() in samsung_early_write()?
> Anyway functions names need to be changes accordingly.

Yes, this is exactly what we had implemented in the patch I mentioned,
except that the putc function was generic for all hardware variants.

Best regards,
Tomasz
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