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

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Date:   Thu, 21 Sep 2017 00:00:01 +0200
From:   Mario Hüttel <mario.huettel@....net>
To:     Franklin S Cooper Jr <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
Subject: Re: [PATCH v2 1/3] can: m_can: Make hclk optional

> 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 also agree in this point.
However, there's a problem I want to point out:

The M_CAN can only function correctly, if the condition
'hclk >= cclk' holds true.

The internal clock domain crossing can fail if this condition
is violated.

I thought about adding the condition to the driver to ensure
correct operation. But I had some problems:

1. Determine the clock rates:
    The devices you've mentioned above don't have an assigned
    hclk. Is there still a possibility to get the clock frequency?

2. What to do if 'hclk < cclk'?
    Is there a general way to process such an error? - I think not.
    Is a simple warning in case of an error enough?

Is there a way of ensuring that the condition is always met at all?

I think it is quite unlikely that the condition is violated, because
devices that actually run Linux usually have (bus) clock rates that
are high enough to ensure the correct operation.

Would it be safe to just ignore this possible fault?

Regards

Mario

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

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