linux-kernel - Re: [PATCH v1 2/3] drivers/perf: add DesignWare PCIe PMU driver

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Date:   Fri, 23 Sep 2022 13:51:41 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     will@...nel.org, Jonathan.Cameron@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        rdunlap@...radead.org, robin.murphy@....com, mark.rutland@....com,
        baolin.wang@...ux.alibaba.com, zhuo.song@...ux.alibaba.com,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v1 2/3] drivers/perf: add DesignWare PCIe PMU driver

On Fri, Sep 23, 2022 at 10:46:09PM +0800, Shuai Xue wrote:
> 在 2022/9/23 AM1:36, Bjorn Helgaas 写道:
> > On Sat, Sep 17, 2022 at 08:10:35PM +0800, Shuai Xue wrote:

> >> +static struct device_attribute dwc_pcie_pmu_cpumask_attr =
> >> +__ATTR(cpumask, 0444, dwc_pcie_pmu_cpumask_show, NULL);
> > 
> > DEVICE_ATTR_RO()?

> DEVICE_ATTR_RO may a good choice. But does it fit the code style to use
> DEVICE_ATTR_RO in drivers/perf? As far as know, CCN, CCI, SMMU,
> qcom_l2_pmu use "struct device_attribute" directly.

DEVICE_ATTR_RO is just newer, and I think CCN, CCI, SMMU, etc. would
be using it if they were written today.  Of course, the drivers/perf
maintainers may have a different opinion :)

> > I think every caller of dwc_pcie_pmu_read_dword() makes the same check
> > and prints the same message; maybe the message should be moved inside
> > dwc_pcie_pmu_read_dword()?
> > 
> > Same with dwc_pcie_pmu_write_dword(); moving the message there would
> > simplify all callers.
> 
> I would like to wrap dwc_pcie_pmu_{write}_dword out, use
> pci_{read}_config_dword and drop the snaity check of return value as
> Jonathan suggests.  How did you like it?

Sounds good.  Not sure the error checking is worthwhile since
pci_read_config_dword() really doesn't return meaningful errors
anyway.

> >> +static struct dwc_pcie_info_table *pmu_to_pcie_info(struct pmu *pmu)
> >> +{
> >> +	struct dwc_pcie_info_table *pcie_info;
> >> +	struct dwc_pcie_pmu *pcie_pmu = to_pcie_pmu(pmu);
> >> +
> >> +	pcie_info = container_of(pcie_pmu, struct dwc_pcie_info_table, pcie_pmu);
> >> +	if (pcie_info == NULL)
> >> +		pci_err(pcie_info->pdev, "Can't get pcie info\n");
> > 
> > It shouldn't be possible to get here for a pmu with no pcie_info, and
> > callers don't check for a NULL pointer return value before
> > dereferencing it, so I guess all this adds is an error message before
> > a NULL pointer oops?  Not sure the code clutter is worth it.
> 
> Do you mean to drop the snaity check of container_of?

Yes.  I'm suggesting that the NULL pointer oops itself has enough
information to debug this problem, even without the pci_err().

Bjorn