On Fri, Aug 11, 2017 at 05:29:57PM +0100, Robin Murphy wrote:
> The good news is that DT already gives us the ammunition to do the right
> thing - anything lacking a "dma-ranges" property should be considered
> not to have a mapping of DMA address space from its children to its
> parent, thus anything for which of_dma_get_range() does not succeed does
> not need DMA configuration.

That sounds like a good heuristic.

At least in theory apparently :(

> The bad news is that strictly enforcing that would likely break just
> about every FDT platform out there, since most authors have either not
> considered the property at all or have mistakenly assumed that omitting
> "dma-ranges" is equivalent to including the empty property. Thus we have
> little choice but to special-case platform, AMBA and PCI devices so they
> continue to receive configuration unconditionally as before. At least
> anything new will have to get it right in future...

That still sounds like a useful compromise.

>       ret = of_dma_get_range(np, &dma_addr, &paddr, &size);
>       if (ret < 0) {
> +             /*
> +              * For legacy reasons, we have to assume some devices need
> +              * DMA configuration regardless of whether "dma-ranges" is
> +              * correctly specified or not.
> +              */
> +             if (!dev_is_pci(dev) &&
> +#ifdef CONFIG_ARM_AMBA
> +                 dev->bus != &amba_bustype &&
> +#endif
> +                 dev->bus != &platform_bus_type)
> +                     return ret == -ENODEV ? 0 : ret;
> +

It would be really nice to have dev_is_amba and dev_is_plaform helpers
to reduce the ifdef mess.

But it should be okay even without that cleanup.

> +     if (!size)
> +             size = max(dev->coherent_dma_mask, dev->coherent_dma_mask + 1);

I find this way to deal with an overflow really odd, but given that it's
just moved around I'm not going to complain.

Reply via email to