Re: [PATCH net v3 2/3] net: stmmac: thead: Get and enable APB clock on initialization

[Drew Fustini]

On Fri, Aug 08, 2025 at 09:36:55AM +0000, Yao Zi wrote:
> It's necessary to adjust the MAC TX clock when the linkspeed changes,
> but it's noted such adjustment always fails on TH1520 SoC, and reading
> back from APB glue registers that control clock generation results in
> garbage, causing broken link.
> 
> With some testing, it's found a clock must be ungated for access to APB
> glue registers. Without any consumer, the clock is automatically
> disabled during late kernel startup. Let's get and enable it if it's
> described in devicetree.
> 
> For backward compatibility with older devicetrees, probing won't fail if
> the APB clock isn't found. In this case, we emit a warning since the
> link will break if the speed changes.
> 
> Fixes: 33a1a01e3afa ("net: stmmac: Add glue layer for T-HEAD TH1520 SoC")
> Signed-off-by: Yao Zi <ziyao@xxxxxxxxxxx>
> Tested-by: Drew Fustini <fustini@xxxxxxxxxx>
> ---
>  drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
> index c72ee759aae5..f2946bea0bc2 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
> @@ -211,6 +211,7 @@ static int thead_dwmac_probe(struct platform_device *pdev)
>  	struct stmmac_resources stmmac_res;
>  	struct plat_stmmacenet_data *plat;
>  	struct thead_dwmac *dwmac;
> +	struct clk *apb_clk;
>  	void __iomem *apb;
>  	int ret;
>  
> @@ -224,6 +225,19 @@ static int thead_dwmac_probe(struct platform_device *pdev)
>  		return dev_err_probe(&pdev->dev, PTR_ERR(plat),
>  				     "dt configuration failed\n");
>  
> +	/*
> +	 * The APB clock is essential for accessing glue registers. However,
> +	 * old devicetrees don't describe it correctly. We continue to probe
> +	 * and emit a warning if it isn't present.
> +	 */
> +	apb_clk = devm_clk_get_enabled(&pdev->dev, "apb");
> +	if (PTR_ERR(apb_clk) == -ENOENT)
> +		dev_warn(&pdev->dev,
> +			 "cannot get apb clock, link may break after speed changes\n");
> +	else if (IS_ERR(apb_clk))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(apb_clk),
> +				     "failed to get apb clock\n");
> +
>  	dwmac = devm_kzalloc(&pdev->dev, sizeof(*dwmac), GFP_KERNEL);
>  	if (!dwmac)
>  		return -ENOMEM;
> -- 
> 2.50.1
> 

Reviewed-by: Drew Fustini <fustini@xxxxxxxxxx>

Thanks for improving the commit description and adding the warning.

Drew