linux-kernel - Re: [PATCH 09/15] cxl/acpi: Determine PCI host bridge's ACPI UID

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Date:   Thu, 1 Sep 2022 11:41:33 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Robert Richter <rrichter@....com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 09/15] cxl/acpi: Determine PCI host bridge's ACPI UID

On Thu, 1 Sep 2022 08:53:36 +0200
Robert Richter <rrichter@....com> wrote:

> On 31.08.22 12:00:27, Jonathan Cameron wrote:
> > On Wed, 31 Aug 2022 10:15:57 +0200
> > Robert Richter <rrichter@....com> wrote:
> >   
> > > The UID is needed to read the RCH's CEDT entry with the RCRB base
> > > address. Determine the host's UID from its ACPI fw node.
> > > 
> > > Signed-off-by: Robert Richter <rrichter@....com>
> > > ---
> > >  drivers/cxl/acpi.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > > index f9cdf23a91a8..b3146b7ae922 100644
> > > --- a/drivers/cxl/acpi.c
> > > +++ b/drivers/cxl/acpi.c
> > > @@ -368,8 +368,20 @@ struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> > >  static int __init cxl_restricted_host_probe(struct platform_device *pdev)
> > >  {
> > >  	struct pci_host_bridge *host = NULL;
> > > +	struct acpi_device *adev;
> > > +	unsigned long long uid = ~0;
> > >  
> > >  	while ((host = cxl_find_next_rch(host)) != NULL) {
> > > +		adev = ACPI_COMPANION(&host->dev);
> > > +		if (!adev || !adev->pnp.unique_id ||
> > > +			(kstrtoull(adev->pnp.unique_id, 10, &uid) < 0))  
> > 
> > There is an acpi_device_uid() accessor function that should probably be
> > used here.  
> 
> That accessor actually does not help really, there is no null pointer
> check for adev. Using it actually adds more complexity since another
> variable is introduced plus you need to look at the function's
> implementation anyway.
> 
> The adev->pnp.unique_id access pattern is widely used in the kernel, I
> don't expect changes in the data struct here.

Ok.

> 
> > Also, should a fialure to convert to an integer (or one within
> > limits) be something we paper over?  Feels like we should fail
> > hard if that happens.  
> 
> This is a real corner case and close to a broken firmware
> implementation. I think current dbg messages are good to find where
> the detection stops.

Hmm. I don't like papering over such bugs as it leads to people not
fixing their bios as early as they otherwise would,
	but fair enough I guess.

> 
> > Admittedly I can't immediately find any spec that states that
> > the _UID should be either integer or under 32 bits...
> > ACPI allows a string and CXL just says it's 4 bytes long.  
> 
> IIRC the UID can be implemented as string or 8 bytes, there is no
> limitation then. That's why the range check below.
Ok.

All queries answered so

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> 
> -Robert
> 
> >   
> > > +			continue;
> > > +
> > > +		dev_dbg(&adev->dev, "host uid: %llu\n", uid);
> > > +
> > > +		if (uid > U32_MAX)
> > > +			continue;
> > > +
> > >  		dev_info(&host->dev, "host supports CXL\n");
> > >  	}
> > >    
> >