RE: [PATCH v6 04/15] PCI: Add pci_find_vsec_capability() to find a specific VSEC

[Gustavo Pimentel]

On Wed, Feb 17, 2021 at 22:27:38, Bjorn Helgaas <helgaas@xxxxxxxxxx> 
wrote:

> [+cc Krzysztof, since he commented on a previous version]
> [+cc Lukas, who previously proposed exactly what I suggest below,
> sorry for repeating.  I think Lukas was right to propose passing in
> the vendor ID because it makes it easier to read the caller.]
> 
> When you post new versions of a series, please cc people who commented
> on previous versions.

Noted.

> 
> On Fri, Feb 12, 2021 at 06:37:39PM +0100, Gustavo Pimentel wrote:
> > Adds another helper to ones that already exist called
> > pci_find_vsec_capability. This helper crawls through the device PCI
> > config space searching for a specific ID on the Vendor-Specific Extended
> > Capabilities section.
> 
>   Add pci_find_vsec_capability() to locate a Vendor-Specific Extended
>   Capability with the specified VSEC ID.
> > 
> > The Vendor-Specific Extended Capability (VSEC) is a special PCI
> > capability (acts like container) defined by PCI-SIG that allows the one
> > or more proprietary capabilities defined by the vendor which aren't
> > standard or shared between the manufactures.
> 
> s/is a special ... by PCI-SIG that//
> s/allows the one/allows one/
> s/the manufactures/manufacturers/ (or maybe "vendors" to match previous use)

Ok, I'll include those changes.

> 
> > Signed-off-by: Gustavo Pimentel <gustavo.pimentel@xxxxxxxxxxxx>
> > ---
> >  drivers/pci/pci.c             | 34 ++++++++++++++++++++++++++++++++++
> >  include/linux/pci.h           |  2 ++
> >  include/uapi/linux/pci_regs.h |  6 ++++++
> >  3 files changed, 42 insertions(+)
> > 
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index b9fecc2..628aa9f 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -693,6 +693,40 @@ u8 pci_find_ht_capability(struct pci_dev *dev, int ht_cap)
> >  EXPORT_SYMBOL_GPL(pci_find_ht_capability);
> >  
> >  /**
> > + * pci_find_vsec_capability - Find a vendor-specific extended capability
> > + * @dev: PCI device to query
> > + * @cap: vendor-specific capability ID code
> > + *
> > + * Typically this function will be called by the PCI driver, which passes
> > + * through argument the 'struct pci_dev *' already pointing for the device
> > + * config space that is associated with the vendor and device ID which will
> > + * know which ID to search and what to do with it, however, there might be
> > + * cases that this function could be called outside of this scope and
> > + * therefore is the caller responsibility to check the vendor and/or
> > + * device ID first.
> 
> This is important because it's a bit subtle.  IIUC, each vendor
> (identified by Vendor ID at 0x00 in config space) can define its own
> VSEC IDs, so it can define up to 2^16 == 64K VSEC structures.
> 
> Of course there's not room for that many in config space; but the
> point is that the vendor chooses its own VSEC IDs and doesn't need to
> coordinate with anybody.
> 
> So a VSEC ID 0x0006 in a Synopsys device (Vendor ID 0x16c3) has
> nothing to do with a VSEC ID 0x0006 in an Intel device (Vendor ID
> 0x8086), and it's up to the caller to make sure it's using the correct
> one.
> 
> I wonder if it would help avoid mistakes if we made the interface look
> like this:
> 
>   u16 pci_find_vsec_capability(struct pci_dev *dev, u16 vendor, int vsec_cap_id)
>   {
>     if (vendor != dev->vendor)
>       return 0;
> 
>     while ((vsec = ...))
>       ...
>   }
> 
> so calls would look like this:
> 
>   vsec = pci_find_vsec_capability(dev, PCI_VENDOR_ID_SYNOPSYS, DW_PCIE_VSEC_DMA_ID);
> 
> which would make it more obvious that DW_PCIE_VSEC_DMA_ID is only
> valid in a Synopsys device.
> 
> The function comment could be something like this:
> 
>   pci_find_vsec_capability - Find a vendor-specific extended capability
>   @dev: PCI device to query
>   @vendor: Vendor ID for which capability is defined
>   @vsec_cap_id: Vendor-specific capability ID
> 
>   If @dev has Vendor ID @vendor, search for a VSEC capability with
>   VSEC ID @vsec_cap_id.  If found, return the capability offset in
>   config space; otherwise return 0.
> 
> Or maybe it's even more subtle than I thought, and I'm missing
> something :)

Okay, seems reasonable.

> 
> > + * Returns the address of the vendor-specific structure that matches the
> > + * requested capability ID code within the device's PCI configuration space
> > + * or 0 if it does not find a match.
> > + */
> > +u16 pci_find_vsec_capability(struct pci_dev *dev, int vsec_cap_id)
> > +{
> > +	u16 vsec = 0;
> > +	u32 header;
> > +
> > +	while ((vsec = pci_find_next_ext_capability(dev, vsec,
> > +						     PCI_EXT_CAP_ID_VNDR))) {
> > +		if (pci_read_config_dword(dev, vsec + PCI_VSEC_HDR,
> > +					  &header) == PCIBIOS_SUCCESSFUL &&
> > +		    PCI_VSEC_CAP_ID(header) == vsec_cap_id)
> > +			return vsec;
> > +	}
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(pci_find_vsec_capability);
> > +
> > +/**
> >   * pci_find_parent_resource - return resource region of parent bus of given
> >   *			      region
> >   * @dev: PCI device structure contains resources to be searched
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index b32126d..da6ab6a 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -1080,6 +1080,8 @@ struct pci_bus *pci_find_next_bus(const struct pci_bus *from);
> >  
> >  u64 pci_get_dsn(struct pci_dev *dev);
> >  
> > +u16 pci_find_vsec_capability(struct pci_dev *dev, int vsec_cap_id);
> 
> Can you put this declaration up with the other "find_capability"
> declarations just a few lines up?

Sure.

> 
> >  struct pci_dev *pci_get_device(unsigned int vendor, unsigned int device,
> >  			       struct pci_dev *from);
> >  struct pci_dev *pci_get_subsys(unsigned int vendor, unsigned int device,
> > diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> > index e709ae8..deae275 100644
> > --- a/include/uapi/linux/pci_regs.h
> > +++ b/include/uapi/linux/pci_regs.h
> > @@ -983,6 +983,12 @@
> >  #define PCI_VSEC_HDR		4	/* extended cap - vendor-specific */
> >  #define  PCI_VSEC_HDR_LEN_SHIFT	20	/* shift for length field */
> >  
> > +/* Vendor-Specific Extended Capabilities */
> > +#define PCI_VSEC_HEADER		4	/* Vendor-Specific Header */
> > +#define  PCI_VSEC_CAP_ID(x)	((x) & 0xffff)
> > +#define  PCI_VSEC_CAP_REV(x)	(((x) >> 16) & 0xf)
> > +#define  PCI_VSEC_CAP_LEN(x)	(((x) >> 20) & 0xfff)
> 
> The VSEC #defines are a mess.  We have already have some duplication
> and inconsistent names.  But I think what you need is already there:
> 
>   /* Vendor-Specific (VSEC, PCI_EXT_CAP_ID_VNDR) */
>   #define PCI_VNDR_HEADER         4       /* Vendor-Specific Header */
>   #define  PCI_VNDR_HEADER_ID(x)  ((x) & 0xffff)
>   #define  PCI_VNDR_HEADER_REV(x) (((x) >> 16) & 0xf)
>   #define  PCI_VNDR_HEADER_LEN(x) (((x) >> 20) & 0xfff)

Indeed, I'll use this instead.

> 
> >  /* SATA capability */
> >  #define PCI_SATA_REGS		4	/* SATA REGs specifier */
> >  #define  PCI_SATA_REGS_MASK	0xF	/* location - BAR#/inline */
> > -- 
> > 2.7.4
> > 

-Gustavo