iflib: add new field: isc_dma_width

[ar_semihalf.com]

Summary

Some DMA controllers are unable to address the full host memory space
and are instead limited to a subset of address range (e.g. 48-bit).

Allow the driver to specify the maximum allowed DMA addressing width
(in bits) for the NIC hardware, by introducing a new field in
if_softc_ctx.

If said field is omitted (set to 0), the lowaddr of DMA window bounds
defaults to BUS_SPACE_MAXADDR.

Submitted by: Artur Rojek <[e-mail]>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.

Test Plan

In order to test, set scctx->isc_dma_width to a non-zero value, e.g. scctx->isc_dma_width = 48.

[gallatin]

I'm shocked that this feature was not present before.

[erj]

sys/net/iflib.h

239

You should validate this value -- what if someone puts in something ridiculous by accident?

[ar_semihalf.com]

Changes:

• provide a sanity check for isc_dma_width,
• introduce a DMA_WIDTH_TO_BUS_LOWADDR macro to correctly handle edge cases where isc_dma_width equals either 0 or the number of bits in BUS_SPACE_MAXADDR