Re: accessing a PCIe register from userspace through kmem or other ways ?

On Tue, Apr 05, 2016 at 08:55:54AM -0700, John Baldwin wrote:
> Mostly I do not have experience with MGTDEVICE, though I was planning to
> look at it as a way to implement this.  Two things though: 1) there may
> not be a cdev to associate with, and 2) I know of at least one device driver
> that would use this in addition to using this for a general "map this BAR"
> ioctl on /dev/pci.
So /dev/pci is the natural cdev to place the functionality.
An ioctl on /dev/pci may mmap BAR and return the base address.

> There are other cases in the past where I used OBJT_SG
> but would have preferred to use a variant that used managed pages so that
> I could invidate any existing mappings.  In particular what I want to do
> is invalidate an object so that any future uses fail.
> 
> Alternatively, it might be nice to hook a destructor call into a VM object
> so that I could know when the object is no longer in use (knowing that all
> its mappings have been destroyed).  When using OBJT_SG objects as aliases
> for other things (memory allocated via contigmalloc or bus_dma or for
> resources like PCI BARs), I could keep a reference count on the original
> "thing" that I increment when creating an OBJT_SG object to return from
> something like d_mmap_single() or the /dev/pci ioctl and drop the reference
> count in the destructor hook for that object.
This is in essence how GEM objects + MGTDEVICE mappings work for i915.

The only bottleneck in the API arrangement is that d_mmap_single() only
gets the offset as the identifying data to construct the mapping.
For /dev/pci, the offset parameter would need to encode d:b:s:f and BAR
index.