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SubjectRe: [PATCH v4 3/3] iommu: dart: Add DART iommu driver
Looks really good! Just a few minor comments. With them addressed,

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

> + Say Y here if you are using an Apple SoC with a DART IOMMU.

Nit: Do we need to spell out "with a DART IOMMU"? Don't all the apple
socs need DART?

> +/*
> + * This structure is used to identify a single stream attached to a domain.
> + * It's used as a list inside that domain to be able to attach multiple
> + * streams to a single domain. Since multiple devices can use a single stream
> + * it additionally keeps track of how many devices are represented by this
> + * stream. Once that number reaches zero it is detached from the IOMMU domain
> + * and all translations from this stream are disabled.
> + *
> + * @dart: DART instance to which this stream belongs
> + * @sid: stream id within the DART instance
> + * @num_devices: count of devices attached to this stream
> + * @stream_head: list head for the next stream
> + */
> +struct apple_dart_stream {
> + struct apple_dart *dart;
> + u32 sid;
> +
> + u32 num_devices;
> +
> + struct list_head stream_head;
> +};

It wasn't obvious to me why we can get away without reference counting.
Looking ahead it looks like we assert locks in each case. Maybe add
that to the comment?

```
> +static void apple_dart_hw_set_ttbr(struct apple_dart *dart, u16 sid, u16 idx,
> + phys_addr_t paddr)
> +{
> + writel(DART_TTBR_VALID | (paddr >> DART_TTBR_SHIFT),
> + dart->regs + DART_TTBR(sid, idx));
> +}
```

Should we be checking alignment here? Something like

BUG_ON(paddr & ((1 << DART_TTBR_SHIFT) - 1));

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