linux-kernel - Re: [PATCH 1/3] PCI: Add support in pci_walk

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Message-ID: <87mto7unw1.wl-maz@kernel.org>
Date:   Mon, 20 Sep 2021 19:01:50 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        <lokeshvutla@...com>
Subject: Re: [PATCH 1/3] PCI: Add support in pci_walk_bus() to invoke callback matching RID

On Mon, 20 Sep 2021 15:28:52 +0100,
Kishon Vijay Abraham I <kishon@...com> wrote:
> 
> Hi Marc,
> 
> On 20/09/21 2:26 pm, Marc Zyngier wrote:
> > On Mon, 20 Sep 2021 07:41:31 +0100,
> > Kishon Vijay Abraham I <kishon@...com> wrote:
> >>
> >> Add two arguments to pci_walk_bus() [requestorID and mask], and add
> >> support in pci_walk_bus() to invoke the *callback* only for devices
> >> whose RequestorID after applying *mask* matches with *requestorID*
> >> passed as argument.
> >>
> >> This is done in preparation for calculating the total number of
> >> interrupt vectors that has to be supported by a specific GIC ITS device ID,
> >> specifically when "msi-map-mask" is populated in device tree.
> >>
> >> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
> >> ---
> >>  drivers/pci/bus.c   | 13 +++++++++----
> >>  include/linux/pci.h |  7 +++++--
> >>  2 files changed, 14 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> >> index 3cef835b375f..e381e639ceaa 100644
> >> --- a/drivers/pci/bus.c
> >> +++ b/drivers/pci/bus.c
> >> @@ -358,10 +358,12 @@ void pci_bus_add_devices(const struct pci_bus *bus)
> >>  }
> >>  EXPORT_SYMBOL(pci_bus_add_devices);
> >>  
> >> -/** pci_walk_bus - walk devices on/under bus, calling callback.
> >> +/** __pci_walk_bus - walk devices on/under bus matching requestor ID, calling callback.
> >>   *  @top      bus whose devices should be walked
> >>   *  @cb       callback to be called for each device found
> >>   *  @userdata arbitrary pointer to be passed to callback.
> >> + *  @rid      Requestor ID that has to be matched for the callback to be invoked
> >> + *  @mask     Mask that has to be applied to pci_dev_id(), before compating it with @rid
> >>   *
> >>   *  Walk the given bus, including any bridged devices
> >>   *  on buses under this bus.  Call the provided callback
> >> @@ -371,8 +373,8 @@ EXPORT_SYMBOL(pci_bus_add_devices);
> >>   *  other than 0, we break out.
> >>   *
> >>   */
> >> -void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >> -		  void *userdata)
> >> +void __pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >> +		    void *userdata, u32 rid, u32 mask)
> >>  {
> >>  	struct pci_dev *dev;
> >>  	struct pci_bus *bus;
> >> @@ -399,13 +401,16 @@ void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >>  		} else
> >>  			next = dev->bus_list.next;
> >>  
> >> +		if (mask != 0xffff && ((pci_dev_id(dev) & mask) != rid))
> > 
> > Why the check for the mask? I also wonder whether the mask should apply
> > to the rid as well:
> 
> If the mask is set for all 16bits, then there is not going to be two PCIe
> devices which gets the same ITS device ID right? So no need for calculating
> total number of vectors?

Are we really arguing about the cost of a compare+branch vs some
readability? Or is there an actual correctness issue here?

> > 
> > 		if ((pci_dev_id(dev) & mask) != (rid & mask))

Because I think the above expression is a lot more readable (and
likely more correct) than what you are suggesting.

> > 
> >> +			continue;
> >> +
> >>  		retval = cb(dev, userdata);
> >>  		if (retval)
> >>  			break;
> >>  	}
> >>  	up_read(&pci_bus_sem);
> >>  }
> >> -EXPORT_SYMBOL_GPL(pci_walk_bus);
> >> +EXPORT_SYMBOL_GPL(__pci_walk_bus);
> >>  
> >>  struct pci_bus *pci_bus_get(struct pci_bus *bus)
> >>  {
> >> diff --git a/include/linux/pci.h b/include/linux/pci.h
> >> index cd8aa6fce204..8500fec56e50 100644
> >> --- a/include/linux/pci.h
> >> +++ b/include/linux/pci.h
> >> @@ -1473,14 +1473,17 @@ const struct pci_device_id *pci_match_id(const struct pci_device_id *ids,
> >>  int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max,
> >>  		    int pass);
> >>  
> >> -void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >> -		  void *userdata);
> >> +void __pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >> +		    void *userdata, u32 rid, u32 mask);
> >>  int pci_cfg_space_size(struct pci_dev *dev);
> >>  unsigned char pci_bus_max_busnr(struct pci_bus *bus);
> >>  void pci_setup_bridge(struct pci_bus *bus);
> >>  resource_size_t pcibios_window_alignment(struct pci_bus *bus,
> >>  					 unsigned long type);
> >>  
> >> +#define pci_walk_bus(top, cb, userdata) \
> >> +	 __pci_walk_bus((top), (cb), (userdata), 0x0, 0xffff)
> > 
> > Please keep this close to the helper it replaces. I also really
> > dislike the use of this raw 0xffff. Don't we already have a named
> > constant that represents the mask for a RID?
> 
> I didn't find one on quick look but let me check.

Worse case, you could create your own.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.