On Wed, Aug 21, 2024 at 3:19 AM Andrea della Porta
[off-list ref] wrote:

Hi Rob,

On 19:16 Tue 20 Aug     , Rob Herring wrote:

quoted

On Tue, Aug 20, 2024 at 04:36:06PM +0200, Andrea della Porta wrote:

quoted

A missing or empty dma-ranges in a DT node implies a 1:1 mapping for dma
translations. In this specific case, rhe current behaviour is to zero out

typo

Fixed, thanks!

quoted

quoted

the entire specifier so that the translation could be carried on as an
offset from zero.  This includes address specifier that has flags (e.g.
PCI ranges).
Once the flags portion has been zeroed, the translation chain is broken
since the mapping functions will check the upcoming address specifier

What does "upcoming address" mean?

Sorry for the confusion, this means "address specifier (with valid flags) fed
to the translating functions and for which we are looking for a translation".
While this address has some valid flags set, it will fail the translation step
since the ranges it is matched against have flags zeroed out by the 1:1 mapping
condition.

quoted

quoted

against mismatching flags, always failing the 1:1 mapping and its entire
purpose of always succeeding.
Set to zero only the address portion while passing the flags through.

Can you point me to what the failing DT looks like. I'm puzzled how
things would have worked for anyone.

The following is a simplified and lightly edited) version of the resulting DT
from RPi5:

 pci@0,0 {
        #address-cells = <0x03>;
        #size-cells = <0x02>;
        ......
        device_type = "pci";
        compatible = "pci14e4,2712\0pciclass,060400\0pciclass,0604";
        ranges = <0x82000000 0x00 0x00   0x82000000 0x00 0x00   0x00 0x600000>;
        reg = <0x00 0x00 0x00   0x00 0x00>;

        ......

        rp1@0 {

What does 0 represent here? There's no 0 address in 'ranges' below.
Since you said the parent is a PCI-PCI bridge, then the unit-address
would have to be the PCI devfn and you are missing 'reg' (or omitted
it).

                #address-cells = <0x02>;
                #size-cells = <0x02>;
                compatible = "simple-bus";

The parent is a PCI-PCI bridge. Child nodes have to be PCI devices and
"simple-bus" is not a PCI device.

The assumption so far with all of this is that you have some specific
PCI device (and therefore a driver). The simple-buses under it are
defined per BAR. Not really certain if that makes sense in all cases,
but since the address assignment is dynamic, it may have to. I'm also
not completely convinced we should reuse 'simple-bus' here or define
something specific like 'pci-bar-bus' or something.

                ranges = <0xc0 0x40000000   0x01 0x00 0x00   0x00 0x400000>;
                dma-ranges = <0x10 0x00   0x43000000 0x10 0x00   0x10 0x00>;
                ......
        };
 };

The pci@0,0 bridge node is automatically created by virtue of
CONFIG_PCI_DYNAMIC_OF_NODES, and has no dma-ranges, hence it implies 1:1 dma
mappings (flags for this mapping are set to zero).  The rp1@0 node has
dma-ranges with flags set (0x43000000). Since 0x43000000 != 0x00 any translation
will fail.

It's possible that we should fill in 'dma-ranges' when making these
nodes rather than supporting missing dma-ranges here.

Rob