Subject: Re: [PATCH kernel v2 1/2] dma: Allow mixing bypass and normal IOMMU operation

From: Christoph Hellwig <[email-masked]>
Date: Tue, 27 Oct 2020 17:48:58 +0100

> +static inline bool dma_handle_direct(struct device *dev, dma_addr_t dma_handle)
> +{
> +       return dma_handle >= dev->archdata.dma_offset;
> +}

This won't compile except for powerpc, and directly accesing arch members
in common code is a bad idea.  Maybe both your helpers need to be
supplied by arch code to better abstract this out.

>  	if (dma_map_direct(dev, ops))
>  		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
> +#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
> +	else if (dev->bus_dma_limit &&
> +		 can_map_direct(dev, (phys_addr_t) page_to_phys(page) + offset + size))
> +		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
> +#endif

I don't think page_to_phys needs a phys_addr_t on the return value.
I'd also much prefer if we make this a little more beautiful, here
are a few suggestions:

 - hide the bus_dma_limit check inside can_map_direct, and provide a
   stub so that we can avoid the ifdef
 - use a better name for can_map_direct, and maybe also a better calling
   convention by passing the page (the sg code also has the page), and
   maybe even hide the dma_map_direct inside it.

	if (dma_map_direct(dev, ops) ||
	    arch_dma_map_page_direct(dev, page, offset, size))
		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);

>  	BUG_ON(!valid_dma_direction(dir));
>  	if (dma_map_direct(dev, ops))
>  		dma_direct_unmap_page(dev, addr, size, dir, attrs);
> +#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
> +	else if (dev->bus_dma_limit && dma_handle_direct(dev, addr + size))
> +		dma_direct_unmap_page(dev, addr, size, dir, attrs);
> +#endif

Same here.

>  	if (dma_map_direct(dev, ops))
>  		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
> +#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
> +	else if (dev->bus_dma_limit) {
> +		struct scatterlist *s;
> +		bool direct = true;
> +		int i;
> +
> +		for_each_sg(sg, s, nents, i) {
> +			direct = can_map_direct(dev, sg_phys(s) + s->offset + s->length);
> +			if (!direct)
> +				break;
> +		}
> +		if (direct)
> +			ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
> +		else
> +			ents = ops->map_sg(dev, sg, nents, dir, attrs);
> +	}
> +#endif

This needs to go into a helper as well.  I think the same style as
above would work pretty nicely as well:

 	if (dma_map_direct(dev, ops) ||
	    arch_dma_map_sg_direct(dev, sg, nents))
 		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
 	else
 		ents = ops->map_sg(dev, sg, nents, dir, attrs);

> +#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
> +	if (dev->bus_dma_limit) {
> +		struct scatterlist *s;
> +		bool direct = true;
> +		int i;
> +
> +		for_each_sg(sg, s, nents, i) {
> +			direct = dma_handle_direct(dev, s->dma_address + s->length);
> +			if (!direct)
> +				break;
> +		}
> +		if (direct) {
> +			dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
> +			return;
> +		}
> +	}
> +#endif

One more time here..