linux-kernel - Re: [PATCH v5 03/16] CXL/PCI: Introduce PCIe helper functions pcie_is_cxl() and pcie_is_cxl

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Message-ID: <Z4eH3izmcBWYwynt@wunner.de>
Date: Wed, 15 Jan 2025 11:03:10 +0100
From: Lukas Wunner <lukas@...ner.de>
To: "Bowman, Terry" <terry.bowman@....com>
Cc: Ira Weiny <ira.weiny@...el.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	nifan.cxl@...il.com, dave@...olabs.net, jonathan.cameron@...wei.com,
	dave.jiang@...el.com, alison.schofield@...el.com,
	vishal.l.verma@...el.com, dan.j.williams@...el.com,
	bhelgaas@...gle.com, mahesh@...ux.ibm.com, oohall@...il.com,
	Benjamin.Cheatham@....com, rrichter@....com,
	nathan.fontenot@....com, Smita.KoralahalliChannabasappa@....com,
	ming.li@...omail.com, PradeepVineshReddy.Kodamati@....com,
	alucerop@....com
Subject: Re: [PATCH v5 03/16] CXL/PCI: Introduce PCIe helper functions
 pcie_is_cxl() and pcie_is_cxl_port()

On Tue, Jan 14, 2025 at 09:19:12AM -0600, Bowman, Terry wrote:
> On 1/13/2025 5:49 PM, Ira Weiny wrote:
> > Terry Bowman wrote:
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -5036,10 +5036,23 @@ static int pci_dev_reset_slot_function(struct pci_dev *dev, bool probe)
> > >  
> > >  static u16 cxl_port_dvsec(struct pci_dev *dev)
> > >  {
> > > +	if (!pcie_is_cxl(dev))
> > > +		return 0;
> > > +
> > >  	return pci_find_dvsec_capability(dev, PCI_VENDOR_ID_CXL,
> > >  					 PCI_DVSEC_CXL_PORT);
> > >  }
> > >  
> > > +bool pcie_is_cxl_port(struct pci_dev *dev)
> > > +{
> > > +	if ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
> > > +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_UPSTREAM) &&
> > > +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM))
> > > +		return false;
> > > +
> > > +	return cxl_port_dvsec(dev);
> > 
> > Returning bool from a function which returns u16 is odd and I don't think
> > it should be coded this way.  I don't think it is wrong right now but this
> > really ought to code the pcie_is_cxl() here and leave cxl_port_dvsec()
> > alone.  Calling cxl_port_dvsec(), checking for if the dvsec exists, and
> > returning bool.
> 
> Thanks for reviewing. Is this what you are looking for here:
> 
> +bool pcie_is_cxl_port(struct pci_dev *dev)
> +{
> +	return (cxl_port_dvsec(dev) > 0);

Since cxl_port_dvsec() cannot return a negative integer,
you might as well use:

	return !!cxl_port_dvsec(dev);

However last I checked gcc generates code which implicitly turns
a number bigger than 1 into a 1 if the return type is bool.
(I had to fix a bug caused by this behavior once, see 009f8c90f571).

Thanks,

Lukas