linux-kernel - Re: [PATCH v7 3/5] Add debugfs based silicon debug support in DWC

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Message-ID: <CAMuHMdVRSjkss3gPnocXpfPQ=mEo4AevpaU=fdGvm=kb3RTmcQ@mail.gmail.com>
Date: Thu, 6 Mar 2025 09:22:34 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Krzysztof Wilczyński <kw@...ux.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, Bjorn Helgaas <helgaas@...nel.org>, 
	Fan Ni <nifan.cxl@...il.com>, Shradha Todi <shradha.t@...sung.com>, 
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-perf-users@...r.kernel.org, 
	lpieralisi@...nel.org, robh@...nel.org, bhelgaas@...gle.com, 
	jingoohan1@...il.com, Jonathan.Cameron@...wei.com, a.manzanares@...sung.com, 
	pankaj.dubey@...sung.com, cassel@...nel.org, 18255117159@....com, 
	xueshuai@...ux.alibaba.com, renyu.zj@...ux.alibaba.com, will@...nel.org, 
	mark.rutland@....com, Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>, 
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v7 3/5] Add debugfs based silicon debug support in DWC

Hi Krzysztof,

On Wed, 5 Mar 2025 at 20:10, Krzysztof Wilczyński <kw@...ux.com> wrote:
> [...]
> > > > Even though debugfs_init() failure is not supposed to fail the probe(),
> > > > dwc_pcie_rasdes_debugfs_init() has a devm_kzalloc() and propagating that
> > > > failure would be canolically correct IMO.
> > >
> > > I'm not sure about this.  What's the requirement to propagate
> > > devm_kzalloc() failures?  I think devres will free any allocs that
> > > were successful regardless.
> > >
> > > IIUC, we resolved the Gray Hawk Single issue by changing
> > > dwc_pcie_rasdes_debugfs_init() to return success without doing
> > > anything when there's no RAS DES Capability.
> > >
> > > But dwc_pcie_debugfs_init() can still return failure, and that still
> > > causes dw_pcie_ep_init_registers() to fail, which breaks the "don't
> > > propagate debugfs issues upstream" rule:
> > >
> > >   int dw_pcie_ep_init_registers(struct dw_pcie_ep *ep)
> > >   {
> > >           ...
> > >           ret = dwc_pcie_debugfs_init(pci);
> > >           if (ret)
> > >                   goto err_remove_edma;
> > >
> > >           return 0;
> > >
> > >   err_remove_edma:
> > >           dw_pcie_edma_remove(pci);
> > >
> > >           return ret;
> > >   }
> > >
> > > We can say that kzalloc() failure should "never" happen, and therefore
> > > it's OK to fail the driver probe if it happens, but that doesn't seem
> > > like a strong argument for breaking the "don't propagate debugfs
> > > issues" rule.  And someday there may be other kinds of failures from
> > > dwc_pcie_debugfs_init().

pcie-designware-debugfs.c only does small allocations.  If any of
these fail, you have much bigger problems, and the system will die soon,
irrespective of propagating the -ENOMEM or not...

Another issue is that the caller does not handle failures correctly,
given (A) the irqdomain WARNING I got, and (B) the half-registered
PCI bus, oopsing on "lspci"...

> > Fine with me. I was not too sure about propagating failure either.
>
> What if we do this?
>
> diff --git i/drivers/pci/controller/dwc/pcie-designware-debugfs.c w/drivers/pci/controller/dwc/pcie-designware-debugfs.c
> index 586a9d107434..fddf71f014c4 100644
> --- i/drivers/pci/controller/dwc/pcie-designware-debugfs.c
> +++ w/drivers/pci/controller/dwc/pcie-designware-debugfs.c
> @@ -162,7 +162,7 @@ void dwc_pcie_debugfs_deinit(struct dw_pcie *pci)
>         debugfs_remove_recursive(pci->debugfs->debug_dir);
>  }
>
> -int dwc_pcie_debugfs_init(struct dw_pcie *pci)
> +void dwc_pcie_debugfs_init(struct dw_pcie *pci)
>  {
>         char dirname[DWC_DEBUGFS_BUF_MAX];
>         struct device *dev = pci->dev;
> @@ -174,17 +174,15 @@ int dwc_pcie_debugfs_init(struct dw_pcie *pci)
>         snprintf(dirname, DWC_DEBUGFS_BUF_MAX, "dwc_pcie_%s", dev_name(dev));
>         dir = debugfs_create_dir(dirname, NULL);
>         debugfs = devm_kzalloc(dev, sizeof(*debugfs), GFP_KERNEL);
> -       if (!debugfs)
> -               return -ENOMEM;
> +       if (!debugfs) {
> +               dev_err(dev, "failed to allocate memory for debugfs\n");

There is no need to print an error message when a memory allocation
fails, as the memory allocation core already takes care of that.
So please drop the dev_err() call.

> +               return;
> +       }
>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds