Re: [PATCH V6 04/10] PCI/DOE: Introduce pci_doe_create_doe_devices

From: Bjorn Helgaas <helgaas@kernel.org>
To: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Cc: ira.weiny@intel.com, Dan Williams <dan.j.williams@intel.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Alison Schofield <alison.schofield@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Ben Widawsky <ben.widawsky@intel.com>,
	linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org,
	linux-pci@vger.kernel.org
Subject: Re: [PATCH V6 04/10] PCI/DOE: Introduce pci_doe_create_doe_devices
Date: Fri, 4 Feb 2022 10:27:56 -0600	[thread overview]
Message-ID: <20220204162756.GA187525@bhelgaas> (raw)
In-Reply-To: <20220204145116.00000f5c@Huawei.com>

On Fri, Feb 04, 2022 at 02:51:16PM +0000, Jonathan Cameron wrote:
> On Thu, 3 Feb 2022 16:44:37 -0600
> Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Mon, Jan 31, 2022 at 11:19:46PM -0800, ira.weiny@intel.com wrote:

> > > + * pci_doe_create_doe_devices - Create auxiliary DOE devices for all DOE
> > > + *                              mailboxes found
> > > + * @pci_dev: The PCI device to scan for DOE mailboxes
> > > + *
> > > + * There is no coresponding destroy of these devices.  This function associates
> > > + * the DOE auxiliary devices created with the pci_dev passed in.  That
> > > + * association is device managed (devm_*) such that the DOE auxiliary device
> > > + * lifetime is always greater than or equal to the lifetime of the pci_dev.  
> > 
> > This seems backwards.  What does it mean if the DOE aux dev
> > lifetime is *greater* than that of the pci_dev?  Surely you can't
> > access a PCI DOE Capability if the pci_dev is gone?
> 
> I think the description is inaccurate - the end of life is the same
> as that of the PCI driver binding to the pci_dev.  It'll get cleared
> up if that is unbound etc.

I don't know much about devm, but I *think* the devm things get
released by devres_release_all(), which is called by
__device_release_driver() after it calls the bus or driver's .remove()
method (pci_device_remove(), in this case).

So in this case, I think the aux dev is created after the pci_dev and
released after the PCI driver and the PCI core are done with the
pci_dev.  I assume some refcounting prevents the pci_dev from actually
being deallocated until the aux dev is done with it.

I'm not confident that this is a robust situation.

> > > +		 * done later within the DOE initialization, but as it
> > > +		 * potentially has other impacts keep it here when setting up
> > > +		 * the IRQ's.  
> > 
> > s/IRQ's/IRQs/
> > 
> > "Potentially has other impacts" is too vague, and this doesn't
> > explain why bus mastering should be enabled here rather than
> > later.  The device should not issue an MSI-X until DOE Interrupt
> > Enable is set, so near there seems like a logical place.
> 
> I can't remember what lead to that comment so hopefully moving to
> just before the enable would be fine - if there was somewhere to do
> it.  I'm not sure there is as the IRQ enable is in the Auxilliary
> Bus driver.  If we pull the pci_alloc_irq_vectors() out of here into
> the caller, then the pci_set_master() should go with it.

I think pci_set_master() is tied to setting PCI_DOE_CTRL_INT_EN, not
to pci_alloc_irq_vectors().

Bjorn