linux-kernel - Re: [PATCH v6 0/4] drivers/perf: add Synopsys DesignWare PCIe PMU driver support

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Date:   Fri, 28 Jul 2023 14:39:27 +0100
From:   Will Deacon <will@...nel.org>
To:     Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        chengyou@...ux.alibaba.com, kaishen@...ux.alibaba.com,
        yangyicong@...wei.com, baolin.wang@...ux.alibaba.com,
        robin.murphy@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
        rdunlap@...radead.org, mark.rutland@....com,
        zhuo.song@...ux.alibaba.com
Subject: Re: [PATCH v6 0/4] drivers/perf: add Synopsys DesignWare PCIe PMU
 driver support

On Thu, Jul 27, 2023 at 11:45:22AM +0800, Shuai Xue wrote:
> 
> 
> On 2023/7/26 04:59, Bjorn Helgaas wrote:
> > On Mon, Jul 24, 2023 at 10:18:07AM +0100, Jonathan Cameron wrote:
> >> On Mon, 24 Jul 2023 10:34:08 +0800
> >> Shuai Xue <xueshuai@...ux.alibaba.com> wrote:
> >>> On 2023/7/10 20:04, Shuai Xue wrote:
> >>>> On 2023/6/16 16:39, Shuai Xue wrote:  
> >>>>> On 2023/6/6 15:49, Shuai Xue wrote:  
> > 
> >>>>>> This patchset adds the PCIe Performance Monitoring Unit (PMU) driver support
> >>>>>> for T-Head Yitian 710 SoC chip. Yitian 710 is based on the Synopsys PCI Express
> >>>>>> Core controller IP which provides statistics feature.
> > 
> >> ...
> >> Really a question for Bjorn I think, but here is my 2 cents...
> >>
> >> The problem here is that we need to do that fundamental redesign of the
> >> way the PCI ports drivers work.  I'm not sure there is a path to merging
> >> this until that is done.  The bigger problem is that I'm not sure anyone
> >> is actively looking at that yet.  I'd like to look at this (as I have
> >> the same problem for some other drivers), but it is behind various
> >> other things on my todo list.
> >>
> >> Bjorn might be persuaded on a temporary solution, but that would come
> >> with some maintenance problems, particularly when we try to do it
> >> 'right' in the future.  Maybe adding another service driver would be
> >> a stop gap as long as we know we won't keep doing so for ever. Not sure.
> > 
> > I think the question here is around the for_each_pci_dev() in
> > __dwc_pcie_pmu_probe()?  I don't *like* that because of the
> > assumptions it breaks (autoload doesn't work, hotplug doesn't work),
> > but:
> > 
> >   - There are several other drivers that also do this,
> >   - I don't have a better suggest for any of them,
> >   - It's not a drivers/pci thing, so not really up to me anyway,
> > 
> > so I don't have any problem with this being merged as-is, as long as
> > you can live with the limitations.
> > 
> > I don't think this series does anything to work around those
> > limitations, i.e., it doesn't make up fake device IDs for module
> > loading or fake events for hotplug, so it seems like we could improve
> > the implementation later if we ever have a way to do it.
> > 
> > Bjorn
> 
> + Will
> 
> Ok, thank you for confirmation, Bjorn. Then it comes to perf driver parts and
> it is really a question for @Will I think.
> 
> What's your opinion about merging this patch set, @Will?

No fundamental objection from me, but I'll have a closer look when you
post a version addressing the feedback from Jonathan and Yicong.

Cheers,

Will