linux-kernel - Re: [PATCH V2 04/11] cxl/mem: Clear events on driver load

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Date:   Fri, 2 Dec 2022 15:34:20 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        "Vishal Verma" <vishal.l.verma@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>
Subject: Re: [PATCH V2 04/11] cxl/mem: Clear events on driver load

Ira Weiny wrote:
> On Thu, Dec 01, 2022 at 06:48:12PM -0800, Dan Williams wrote:
> > cxl/mem is cxl_mem.ko, This is cxl/pci.
> > 
> > ira.weiny@ wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> > > The information contained in the events prior to the driver loading can
> > > be queried at any time through other mailbox commands.
> > > 
> > > Ensure a clean slate of events by reading and clearing the events.  The
> > > events are sent to the trace buffer but it is not anticipated to have
> > > anyone listening to it at driver load time.
> > 
> > This is easy to guarantee with modprobe policy, so I am not sure it is
> > worth stating.
> 
> Fair enough.  But there was some discussion early on regarding why reading and
> clearing on startup was a good thing.  This showed that we chose to do that and
> why we don't care.  I'll remove it.
> 
> > 
> > This breakdown feels odd. I would split the trace event definitions into
> > its own lead in patch since that is a pile of definitions that can be
> > merged on their own. Then squash get, clear, and this patch into one
> > patch as they don't have much reason to go in separately.
> 
> I agree that splitting the Get/Clear/and this patch was odd.  However,
> splitting Get/Clear made the discussion on those operations easier IMO.
> 
> As a result this did not really belong in either of those patches on their own.
> 
> It is also very clearly a do one thing per patch situation.
> 
> > 
> > > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > > Reviewed-by: Dave Jiang <dave.jiang@...el.com>
> > > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > > ---
> > >  drivers/cxl/pci.c            | 2 ++
> > >  tools/testing/cxl/test/mem.c | 2 ++
> > >  2 files changed, 4 insertions(+)
> > > 
> > > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > > index 8f86f85d89c7..11e95a95195a 100644
> > > --- a/drivers/cxl/pci.c
> > > +++ b/drivers/cxl/pci.c
> > > @@ -521,6 +521,8 @@ static int cxl_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
> > >  	if (IS_ERR(cxlmd))
> > >  		return PTR_ERR(cxlmd);
> > >  
> > > +	cxl_mem_get_event_records(cxlds);
> > > +
> > >  	if (resource_size(&cxlds->pmem_res) && IS_ENABLED(CONFIG_CXL_PMEM))
> > >  		rc = devm_cxl_add_nvdimm(&pdev->dev, cxlmd);
> > >  
> > > diff --git a/tools/testing/cxl/test/mem.c b/tools/testing/cxl/test/mem.c
> > > index aa2df3a15051..e2f5445d24ff 100644
> > > --- a/tools/testing/cxl/test/mem.c
> > > +++ b/tools/testing/cxl/test/mem.c
> > > @@ -285,6 +285,8 @@ static int cxl_mock_mem_probe(struct platform_device *pdev)
> > >  	if (IS_ERR(cxlmd))
> > >  		return PTR_ERR(cxlmd);
> > >  
> > > +	cxl_mem_get_event_records(cxlds);
> > > +
> > 
> > This hunk likely goes with the first patch that actually implements some
> > mocked events.
> 
> If this patch was squashed into the other patches yes.  But as a patch which
> does exactly 1 thing "Clear events on driver load" it works IMO.  I could just
> have well put this patch at the very end.
> 
> Now that the Get/Clear operations are more settled I'll split this out and
> squash it as you suggest.  Jonathan suggested squashing Get/Clear too but again
> I really prefer the 1 thing/patch and each of those operations seemed like a
> good breakdown.
> 

I'll preface this by saying if you ask 3 kernel developers how to split
a patch series you'll get 5 answers. For me though, a patch should be a
bisectable full-thought. That at each step of a series the kernel is
incrementally better in a way that makes sense. The kernel that gets Get
Events likely needs to clear them too to complete 1 full thought about
enbling Event handling. Otherwise a kernel that just retrieves some
events until they overflow feels like a POC.