Re: Legacy features in PCI Express devices

On Mon, Mar 13, 2017 at 05:10:57PM +0100, Mason wrote:

Hello,

There are two revisions of our PCI Express controller.

Rev 1 did not support the following features:

  1) legacy PCI interrupt delivery (INTx signals)
  2) I/O address space

Internally, someone stated that such missing support would prevent
some PCIe cards from working with our controller.

Are there really modern PCIe cards that require 1) and/or 2)
to function?

From a spec point of view, all endpoints, including Legacy, PCI

Express, and Root Complex Integrated Endpoints, are "required to
support MSI or MSI-X or both if an interrupt resource is requested"
(PCIe r3.0, sec 1.3.2).

The same section says Legacy Endpoints are permitted to use I/O
Requests, but PCI Express and Root Complex Integrated Endpoints are
not.  There's a little wiggle room in the I/O BAR description; I would
interpret it to mean the latter two varieties are permitted to have
I/O BARs, but they must make the resources described by those BARs
available via a memory BAR as well, so they can operate with I/O
address space.

But that's only in theory; David has already given examples of devices
that don't support MSI, and Greg hinted at new devices that might
require I/O space.  I'm particularly curious about that last one,
because there are several host bridges that don't support I/O space at
all.

Bjorn