Subject: Re: [PATCH] ARM: mach-imx6q: add ksz9131rn_phy_fixup

From: Russell King - ARM Linux admin <[email-masked]>
Date: Thu, 5 Mar 2020 13:53:46 +0000

On Thu, Mar 05, 2020 at 02:49:28PM +0100, Philippe Schenker wrote:
> The MAC of the i.MX6 SoC is compliant with RGMII v1.3. The KSZ9131 PHY
> is like KSZ9031 adhering to RGMII v2.0 specification. This means the
> MAC should provide a delay to the TXC line. Because the i.MX6 MAC does
> not provide this delay this has to be done in the PHY.
> 
> This patch adds by default ~1.6ns delay to the TXC line. This should
> be good for all boards that have the RGMII signals routed with the
> same length.
> 
> The KSZ9131 has relatively high tolerances on skew registers from
> MMD 2.4 to MMD 2.8. Therefore the new DLL-based delay of 2ns is used
> and then as little as possibly subtracted from that so we get more
> accurate delay. This is actually needed because the i.MX6 SoC has
> an asynchron skew on TXC from -100ps to 900ps, to get all RGMII
> values within spec.
> 
> Signed-off-by: Philippe Schenker <[email-masked]>
> 
> ---
> 
>  arch/arm/mach-imx/mach-imx6q.c | 37 ++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/arch/arm/mach-imx/mach-imx6q.c b/arch/arm/mach-imx/mach-imx6q.c
> index edd26e0ffeec..8ae5f2fa33e2 100644
> --- a/arch/arm/mach-imx/mach-imx6q.c
> +++ b/arch/arm/mach-imx/mach-imx6q.c
> @@ -61,6 +61,14 @@ static void mmd_write_reg(struct phy_device *dev, int device, int reg, int val)
>  	phy_write(dev, 0x0e, val);
>  }
>  
> +static int mmd_read_reg(struct phy_device *dev, int device, int reg)
> +{
> +	phy_write(dev, 0x0d, device);
> +	phy_write(dev, 0x0e, reg);
> +	phy_write(dev, 0x0d, (1 << 14) | device);
> +	return phy_read(dev, 0x0e);
> +}

These look like the standard MII MMD registers, and it also looks like
you're reinventing phy_read_mmd() - but badly due to lack of locking.

I guess you need this because phy_read_mmd() may be modular - maybe
we should arrange for the accessors to be separately buildable into
the kernel, so that such fixups can stop badly reinventing the wheel?

> +
>  static int ksz9031rn_phy_fixup(struct phy_device *dev)
>  {
>  	/*
> @@ -74,6 +82,33 @@ static int ksz9031rn_phy_fixup(struct phy_device *dev)
>  	return 0;
>  }
>  
> +#define KSZ9131_RXTXDLL_BYPASS	12
> +
> +static int ksz9131rn_phy_fixup(struct phy_device *dev)
> +{
> +	int tmp;
> +
> +	tmp = mmd_read_reg(dev, 2, 0x4c);
> +	/* disable rxdll bypass (enable 2ns skew delay on RXC) */
> +	tmp &= ~(1 << KSZ9131_RXTXDLL_BYPASS);
> +	mmd_write_reg(dev, 2, 0x4c, tmp);
> +
> +	tmp = mmd_read_reg(dev, 2, 0x4d);
> +	/* disable txdll bypass (enable 2ns skew delay on TXC) */
> +	tmp &= ~(1 << KSZ9131_RXTXDLL_BYPASS);
> +	mmd_write_reg(dev, 2, 0x4d, tmp);
> +
> +	/*
> +	 * Subtract ~0.6ns from txdll = ~1.4ns delay.
> +	 * leave RXC path untouched
> +	 */
> +	mmd_write_reg(dev, 2, 4, 0x007d);
> +	mmd_write_reg(dev, 2, 6, 0xdddd);
> +	mmd_write_reg(dev, 2, 8, 0x0007);
> +
> +	return 0;
> +}
> +
>  /*
>   * fixup for PLX PEX8909 bridge to configure GPIO1-7 as output High
>   * as they are used for slots1-7 PERST#
> @@ -167,6 +202,8 @@ static void __init imx6q_enet_phy_init(void)
>  				ksz9021rn_phy_fixup);
>  		phy_register_fixup_for_uid(PHY_ID_KSZ9031, MICREL_PHY_ID_MASK,
>  				ksz9031rn_phy_fixup);
> +		phy_register_fixup_for_uid(PHY_ID_KSZ9131, MICREL_PHY_ID_MASK,
> +				ksz9131rn_phy_fixup);
>  		phy_register_fixup_for_uid(PHY_ID_AR8031, 0xffffffef,
>  				ar8031_phy_fixup);
>  		phy_register_fixup_for_uid(PHY_ID_AR8035, 0xffffffef,
> -- 
> 2.25.1
> 
> 

-- 
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