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

[Drew Fustini <fustini@kernel.org>]

On Mon, Aug 04, 2025 at 09:23:00AM -0700, Drew Fustini wrote:
> On Mon, Aug 04, 2025 at 05:12:26AM +0000, Yao Zi wrote:
> > 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@disroot.org>
> > > > Reviewed-by: Drew Fustini <fustini@kernel.org>
> > > > Tested-by: Drew Fustini <fustini@kernel.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.
> 
> I've just read Conor's comment in the v1 again: 
> 
>  Nah, introduce the warnings. If the clock is required for operation, it
>  should be marked as such. You've made it optional in the driver, which
>  is the important part (backwards compatible) and you've got the dts
>  patch in the series.
> 
> Thus I think the argument I made in my reply to this thread is
> incorrect and the driver should remain backwards compatible.
> 
> > 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.
> 
> I think a comment in the code about why it uses the optional variant of
> the function is a good idea.

I was chatting on devicetree irc channel, and I think that it would be
good to add something like this to the commit message:

 For the purposes of backwards compatibility, the probe will not fail if
 'apb' is not found, but the link will break if the speed changes after
 probe.

Also, if devm_clk_get_optional_enabled("apb") fails, then I think it
would be a good idea to warn the user that changing the link speed will
not be supported.

Thanks,
Drew