RE: [PATCH v7 5/5] Add debugfs based statistical counter support in DWC

Shradha Todi
2025-03-08T15:59:15Z

> -----Original Message-----
> From: Fan Ni <nifan.cxl@gmail.com>
> Sent: 05 March 2025 09:56
> To: Shradha Todi <shradha.t@samsung.com>
> Cc: 'Fan Ni' <nifan.cxl@gmail.com>; 'Krzysztof Wilczyński' <kw@linux.com>; linux-kernel@vger.kernel.org; linux-pci@vger.kernel.org;
> linux-arm-kernel@lists.infradead.org; linux-perf-users@vger.kernel.org; manivannan.sadhasivam@linaro.org; lpieralisi@kernel.org;
> robh@kernel.org; bhelgaas@google.com; jingoohan1@gmail.com; Jonathan.Cameron@huawei.com; a.manzanares@samsung.com;
> pankaj.dubey@samsung.com; cassel@kernel.org; 18255117159@163.com; xueshuai@linux.alibaba.com; renyu.zj@linux.alibaba.com;
> will@kernel.org; mark.rutland@arm.com
> Subject: Re: [PATCH v7 5/5] Add debugfs based statistical counter support in DWC
>
> On Tue, Mar 04, 2025 at 10:40:43PM +0530, Shradha Todi wrote:
> >
> >
> > > -----Original Message-----
> > > From: Fan Ni <nifan.cxl@gmail.com>
> > > Sent: 04 March 2025 02:33
> > > To: Krzysztof Wilczyński <kw@linux.com>
> > > Cc: Fan Ni <nifan.cxl@gmail.com>; Shradha Todi
> > > <shradha.t@samsung.com>; linux-kernel@vger.kernel.org; linux-
> > > pci@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
> > > linux-perf-users@vger.kernel.org; manivannan.sadhasivam@linaro.org;
> > > lpieralisi@kernel.org; robh@kernel.org; bhelgaas@google.com;
> > > jingoohan1@gmail.com; Jonathan.Cameron@huawei.com;
> > > a.manzanares@samsung.com; pankaj.dubey@samsung.com;
> > > cassel@kernel.org; 18255117159@163.com; xueshuai@linux.alibaba.com;
> > > renyu.zj@linux.alibaba.com; will@kernel.org; mark.rutland@arm.com
> > > Subject: Re: [PATCH v7 5/5] Add debugfs based statistical counter
> > > support in DWC
> > >
> > > On Tue, Mar 04, 2025 at 04:42:28AM +0900, Krzysztof Wilczyński wrote:
> > > > Hello,
> > > >
> > > > [...]
> > > > > > +static ssize_t counter_value_read(struct file *file, char
> > > > > > +__user *buf, size_t count, loff_t *ppos) {
> > > > > > + struct dwc_pcie_rasdes_priv *pdata = file->private_data;
> > > > > > + struct dw_pcie *pci = pdata->pci;
> > > > > > + struct dwc_pcie_rasdes_info *rinfo = pci->debugfs->rasdes_info;
> > > > > > + char debugfs_buf[DWC_DEBUGFS_BUF_MAX];
> > > > > > + ssize_t pos;
> > > > > > + u32 val;
> > > > > > +
> > > > > > + mutex_lock(&rinfo->reg_event_lock);
> > > > > > + set_event_number(pdata, pci, rinfo);
> > > > > > + val = dw_pcie_readl_dbi(pci, rinfo->ras_cap_offset + RAS_DES_EVENT_COUNTER_DATA_REG);
> > > > > > + mutex_unlock(&rinfo->reg_event_lock);
> > > > > > + pos = scnprintf(debugfs_buf, DWC_DEBUGFS_BUF_MAX, "Counter
> > > > > > +value: %d\n", val);
> > > > > > +
> > > > > > + return simple_read_from_buffer(buf, count, ppos,
> > > > > > +debugfs_buf, pos); }
> > > > >
> > > > > Do we need to check whether the counter is enabled or not for
> > > > > the event before retrieving the counter value?
> > > >
> > > > I believe, we have a patch that aims to address, have a look at:
> > > >
> > > >
> > > > https://lore.kernel.org/linux-pci/20250225171239.19574-1-manivanna
> > > > n.sa
> > > > dhasivam@linaro.org
> > >
> > > Maybe I missed something, that seems to fix counter_enable_read(), but here is to retrieve counter value.
> > > How dw_pcie_readl_dbi() can return something like "Counter Disabled"?
> > >
> > > Fan
> >
> > Hey Fan,
> > So the counter value will show 0 in case it is disabled so there will
> > not be any issues as per say. We could add the check here but I feel I
> > have already exposed the functionality to check if a counter is enabled or disabled, (by reading the counter_enable debugfs entry)
> so this could be handled in user space to only read the counter if it's enabled.
> Ok.
> Returning 0 when the counter is disabled makes sense to me.
>
> Just some thought.
>
> It seems natural to me if we make "counter_value" only visiable to users when the counter is enabled.
>

Hey Fan,

This looks like a good suggestion to me. I have implemented this and in the process of testing. Since there is no support in the
debugfs framework for conditionally hiding certain files from user, the custom implementation is a little tricky and will need some
discussion before taking in. So let's take this up as a top up patch and live with returning 0 when the counter is disabled for now?

Thanks a lot for the suggestion.
Shradha

> Fan
> >
> > > >
> > > > Thank you!
> > > >
> > > > Krzysztof
> >
> >