linux-kernel - Re: [PATCH net v2 2/3] net: stmmac: thead: Get and enable APB clock on initialization

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Message-ID: <aJBBOptU4IXilK3I@pie>
Date: Mon, 4 Aug 2025 05:12:26 +0000
From: Yao Zi <ziyao@...root.org>
To: Rob Herring <robh@...nel.org>
Cc: Drew Fustini <fustini@...nel.org>, Guo Ren <guoren@...nel.org>,
	Fu Wei <wefu@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Emil Renner Berthing <emil.renner.berthing@...onical.com>,
	Jisheng Zhang <jszhang@...nel.org>, linux-riscv@...ts.infradead.org,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 2/3] net: stmmac: thead: Get and enable APB clock
 on initialization

On Sun, Aug 03, 2025 at 12:02:06PM -0500, Rob Herring wrote:
> On Fri, Aug 01, 2025 at 09:12:39AM +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.
> > 
> > Fixes: 33a1a01e3afa ("net: stmmac: Add glue layer for T-HEAD TH1520 SoC")
> > Signed-off-by: Yao Zi <ziyao@...root.org>
> > Reviewed-by: Drew Fustini <fustini@...nel.org>
> > Tested-by: Drew Fustini <fustini@...nel.org>
> > ---
> >  drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
> > index c72ee759aae5..95096244a846 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 +225,11 @@ static int thead_dwmac_probe(struct platform_device *pdev)
> >  		return dev_err_probe(&pdev->dev, PTR_ERR(plat),
> >  				     "dt configuration failed\n");
> >  
> > +	apb_clk = devm_clk_get_optional_enabled(&pdev->dev, "apb");
> 
> The description sounds like this should not be optional. The binding 
> change also makes it not optional.

Yes, it shouldn't be. But using the non-optional API will cause the
driver fails to probe with the old (problematic) devicetree, IOW, it
breaks the ABI. Comparing to unusable ethernet, failing to adjust the
link speed sounds a minor point to me.

Maybe we could add a comment to explain why optional API is used, or
just use the non-optional one if such ABI breakages are acceptable --
for which I'd like to wait for more opinions.

> Rob
> 

Thanks,
Yao Zi