Subject: Re: [PATCH V4 3/3] can: m_can: add missing message RAM initialization

From: Marc Kleine-Budde <[email-masked]>
Date: Fri, 07 Nov 2014 11:30:38 +0100

On 11/07/2014 09:45 AM, Dong Aisheng wrote:
> The M_CAN message RAM is usually equipped with a parity or ECC functionality.
> But RAM cells suffer a hardware reset and can therefore hold arbitrary
> content at startup - including parity and/or ECC bits.
> 
> To prevent the M_CAN controller detecting checksum errors when reading
> potentially uninitialized TX message RAM content to transmit CAN
> frames the TX message RAM has to be written with (any kind of) initial
> data.
> 
> Signed-off-by: Dong Aisheng <[email-masked]>
> ---
> ChangeLog:
> v3-v4:
>  * initialize the entire Message RAM in use instead of only 8 bytes
>    since this is a valid and needed initialization process.
>    Patch title also changed.
> ---
>  drivers/net/can/m_can/m_can.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index eee1533..de742d0 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1114,7 +1114,7 @@ static int m_can_of_parse_mram(struct platform_device *pdev,
>  	struct resource *res;
>  	void __iomem *addr;
>  	u32 out_val[MRAM_CFG_LEN];
> -	int ret;
> +	int i, start, end, ret;
>  
>  	/* message ram could be shared */
>  	res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "message_ram");
> @@ -1165,6 +1165,15 @@ static int m_can_of_parse_mram(struct platform_device *pdev,
>  		priv->mcfg[MRAM_TXE].off, priv->mcfg[MRAM_TXE].num,
>  		priv->mcfg[MRAM_TXB].off, priv->mcfg[MRAM_TXB].num);
>  
> +	/* initialize the entire Message RAM in use to avoid possible
> +	 * ECC/parity checksum errors when reading an uninitialized buffer
> +	 */
> +	start = priv->mcfg[MRAM_SIDF].off;
> +	end = priv->mcfg[MRAM_TXB].off +
> +		priv->mcfg[MRAM_TXB].num * TXB_ELEMENT_SIZE;
> +	for (i = start; i < end; i += 4)
> +		writel(0x0, priv->mram_base + i);

Can we access the mram without turning on the clock?

Marc

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