On Thu, Jun 02, 2016 at 05:06:55PM +0200, Arnd Bergmann wrote:

On Thursday, June 2, 2016 9:00:01 AM CEST Bjorn Helgaas wrote:

quoted

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I just did a count of the implementations of pci_ops: I found 107
instances of 'struct pci_ops', and 67 of them treat type0 and type1
access differently in some form.

I'd estimate that about half of them, or roughly a third of the total
instances would benefit from my change, if we were to do them again.
Clearly there is no need to change the existing code here when it works,
unless the benefit is very clear and the code is actively maintained.

In some cases, the difference is only that the root bus has a limited
set of devices that are allowed to be accessed, so there would
likely be no benefit of this, compared to e.g. yet another callback
that checks the validity.
Some other instances have type0 registers at a different memory location
from type1, some use different layout inside of that space, and some
are completely different.

The type0/type1 distinction still seems out of place to me at the call
site.  Is there any other reason a caller would care about the
difference between type0 and type1?

The callers really shouldn't care, but they also shouldn't call the
pci_ops function pointer (and as we found earlier, there are only
three such callers).

The distinction between type0 and type1 in my mind is an implementation
detail of the pci_{read,write}_config_{byte,word,dword} functions
that call the low-level operations here.

The caller is performing one abstract operation: reading or writing
config space of a PCI device.  In the caller's context, it makes no
difference whether it's a type0 or type1 access.

Moving the test from the host bridge driver to pci_read_config_byte()
does move a little code from the callee to the caller, and there are
more callees than callers, so in that sense it does remove code
overall.  If you consider a single driver by itself, I'm not sure it
makes much difference.

The pcie-designware.c patch is a net removal of 17 lines, but that's
not all from moving the type0/type1 test: restructuring along the same
lines but keeping the original type0/type1 test removes about 9 lines.

Anyway, I think I'd rather work first on your RFC patches to make
pci_host_bridge the primary structure for probing PCI.  I think
there will be a *lot* of benefit there.

Bjorn