linux-kernel - Re: [PATCH v3] i2c: rk3x: fix bug that cause transfer fails in master receive mode

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Date:	Fri, 22 Aug 2014 09:16:41 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Addy Ke <addy.ke@...k-chips.com>
Cc:	wsa@...-dreams.de, max.schwarz@...ine.de, heiko@...ech.de,
	olof@...om.net, dianders@...omium.org,
	sergei.shtylyov@...entembedded.com, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, cf@...k-chips.com,
	xjq@...k-chips.com, huangtao@...k-chips.com, zyw@...k-chips.com,
	yzq@...k-chips.com, hj@...k-chips.com, kever.yang@...k-chips.com,
	hl@...k-chips.com, caesar.wang@...k-chips.com,
	zhengsq@...k-chips.com
Subject: Re: [PATCH v3] i2c: rk3x: fix bug that cause transfer fails in
 master receive mode

Hi Addy,

On Fri, Aug 22, 2014 at 09:13:11AM +0800, Addy Ke wrote:
> In rk3x SOC, the I2C controller can receive/transmit up to 32 bytes data
> in one chunk, so the size of data to be write/read to/from TXDATAx/RXDATAx
> must be less than or equal 32 bytes at a time.
> 
> Tested on rk3288-pinky board, elan receive 158 bytes data.
> 
> Acked-by: Max Schwarz <max.schwarz@...ine.de>
> Signed-off-by: Addy Ke <addy.ke@...k-chips.com>
> ---
> Changes in v2:
> - Use cleaner syntax as suggested by Sergei.
> - Update commit message as suggested by Wolfram.
> 
> Changes in v3:
> - fix typo: maste --> master and double spaces after 'len'
> 
>  drivers/i2c/busses/i2c-rk3x.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
> index 69e1185..806724a 100644
> --- a/drivers/i2c/busses/i2c-rk3x.c
> +++ b/drivers/i2c/busses/i2c-rk3x.c
> @@ -323,6 +323,10 @@ static void rk3x_i2c_handle_read(struct rk3x_i2c *i2c, unsigned int ipd)
>  	/* ack interrupt */
>  	i2c_writel(i2c, REG_INT_MBRF, REG_IPD);
>  
> +	/* Can only handle a maximum of 32 bytes at a time */
> +	if (unlikely(len > 32))
> +		len = 32;

I do not think we should be using unlikely here (and in another spot in the
driver): unlikely should be used only if the condition happens extremely
infrequently in all of the systems. Here, if a box happens to have a device
that uses transfers larger than 32 bytes we'd be hitting unlikely branch pretty
often.

Thanks.

-- 
Dmitry
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