Re: [PATCH net-next v7 03/14] net: microchip: fdma: add PCIe ATU support

On Fri, Sep 18, 2026 at 01:33:55PM +0200, Daniel Machon wrote:

When lan966x or lan969x operates as a PCIe endpoint, the internal FDMA
engine cannot directly access host memory. Instead, DMA addresses must
be translated through the PCIe Address Translation Unit (ATU). The ATU
provides outbound windows that map internal addresses to PCIe bus
addresses.

The ATU outbound address space (0x10000000-0x1fffffff) is divided into
six equally-sized regions (~42MB each). When FDMA buffers are allocated,
a free ATU region is claimed and programmed with the DMA target address.
The FDMA engine then uses the region's base address in its descriptors,
and the ATU translates these to the actual DMA addresses on the PCIe bus.

Add the required functions and helpers that combine the DMA allocation
with the ATU region mapping. These are used by the lan966x PCIe FDMA
path.

The ATU cannot express a limit finer than its 64KB region granularity,
so pad the mapped allocation to that boundary; otherwise the outbound
window would extend past the memory the host allocated for DMA.

This implementation will also be used by the lan969x, when PCIe FDMA is
added for that platform in the future.

Tested-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>

Reviewed-by: Simon Horman <horms@kernel.org>