From: Alexey Kardashevskiy <redacted>
Sent: Friday, July 8, 2022 5:46 PM

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In general, is "domain" something from hardware or it is a software
concept? Thanks,

'domain' is a software concept to represent the hardware I/O page
table. A blocking domain means all DMAs from a device attached to
this domain are blocked/rejected (equivalent to an empty I/O page
table), usually enforced in the .attach_dev() callback.

Yes, a comment for why having a NULL .attach_dev() is OK is welcomed.

Making it NULL makes __iommu_attach_device() fail, .attach_dev() needs
to return 0 in this crippled environment. Thanks for explaining the
rest, good food for thoughts.

Yeah, that's a typo. 😊