netdev - Re: [v2,1/3] can: m_can: Make hclk optional

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Date:   Thu, 24 Aug 2017 13:30:30 +0530
From:   Sekhar Nori <nsekhar@...com>
To:     Franklin Cooper <fcooper@...com>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-can@...r.kernel.org>, <wg@...ndegger.com>,
        <mkl@...gutronix.de>, <robh+dt@...nel.org>,
        <quentin.schulz@...e-electrons.com>
CC:     "Kristo, Tero" <t-kristo@...com>, Tony Lindgren <tony@...mide.com>,
        Linux OMAP List <linux-omap@...r.kernel.org>
Subject: Re: [v2,1/3] can: m_can: Make hclk optional

+ some OMAP folks and Linux OMAP list

On Tuesday 25 July 2017 04:21 AM, Franklin Cooper wrote:
> Hclk is the MCAN's interface clock. However, for OMAP based devices such as
> DRA7 SoC family the interface clock is handled by hwmod. Therefore, this
> interface clock is managed by hwmod driver via pm_runtime_get and
> pm_runtime_put calls. Therefore, this interface clock isn't defined in DT
> and thus the driver shouldn't fail if this clock isn't found.

I agree that hclk is defined as interface clock for M_CAN IP on DRA76x.

However, there may be a need for the driver to know the value of hclk to
properly configure the RAM watchdog register which has a counter
counting down using hclk. Looks like the driver does not use the RAM
watchdog today. But if there is a need to configure it in future, it
might be a problem.

Is there a restriction in OMAP architecture against passing the
interface clock also in the 'clocks' property in DT. I have not tried it
myself, but wonder if you hit an issue that led to this patch.

> 
> Signed-off-by: Franklin S Cooper Jr <fcooper@...com>
> ---
> Version 2 changes:
> Used NULL instead of 0 for unused hclk handle
> 
>  drivers/net/can/m_can/m_can.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index f4947a7..ea48e59 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1568,8 +1568,13 @@ static int m_can_plat_probe(struct platform_device *pdev)
>  	hclk = devm_clk_get(&pdev->dev, "hclk");
>  	cclk = devm_clk_get(&pdev->dev, "cclk");
>  
> -	if (IS_ERR(hclk) || IS_ERR(cclk)) {
> -		dev_err(&pdev->dev, "no clock found\n");
> +	if (IS_ERR(hclk)) {
> +		dev_warn(&pdev->dev, "hclk could not be found\n");
> +		hclk = NULL;

What is the purpose of NULL setting the clock. I think this is taking it
into a very implementation defined territory and the result could be
different on different architectures. See Russell's explanation here:
https://lkml.org/lkml/2016/11/10/799

Thanks,
Sekhar

> +	}
> +
> +	if (IS_ERR(cclk)) {
> +		dev_err(&pdev->dev, "cclk could not be found\n");
>  		ret = -ENODEV;
>  		goto failed_ret;
>  	}
>