linux-kernel - Re: [PATCH v3 08/24] C6X: memory management and DMA support

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Date:	Wed, 28 Sep 2011 15:46:32 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mark Salter <msalter@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 08/24] C6X: memory management and DMA support

On Tuesday 27 September 2011, Mark Salter wrote:

> +dma_addr_t dma_map_single(struct device *dev, void *ptr, size_t size,
> +			  enum dma_data_direction dir)
> +{
> +	dma_addr_t addr = virt_to_phys(ptr);
> +
> +	BUG_ON(!valid_dma_direction(dir));
> +
> +	switch (dir) {
> +	case DMA_FROM_DEVICE:
> +		L2_cache_block_invalidate(addr, addr + size);
> +		break;
> +	case DMA_TO_DEVICE:
> +		L2_cache_block_writeback(addr, addr + size);
> +		break;
> +	case DMA_BIDIRECTIONAL:
> +		/*
> +		 * If area will be used bidirectionally, syncing will
> +		 * take place manually in the driver.
> +		 */

I think this comment is not strictly correct and you have to do a
writeback+invalidate here. When you map an area, it initially
belongs to the device, until you either do an unmap or sync_*_for_cpu.

> +void dma_unmap_single(struct device *dev, dma_addr_t handle,
> +		      size_t size, enum dma_data_direction dir)
> +{
> +	/* nothing to do here */
> +	debug_dma_unmap_page(dev, handle, size, dir, true);
> +}
> +EXPORT_SYMBOL(dma_unmap_single);

> +void dma_sync_single_for_cpu(struct device *dev, dma_addr_t handle,
> +			     size_t size, enum dma_data_direction dir)
> +{
> +	unsigned long paddr = handle;
> +
> +	BUG_ON(!valid_dma_direction(dir));
> +
> +	switch (dir) {
> +	case DMA_FROM_DEVICE:
> +		L2_cache_block_invalidate(paddr, paddr + size);
> +		break;
> +	case DMA_TO_DEVICE:
> +		L2_cache_block_writeback(paddr, paddr + size);
> +		break;
> +	case DMA_BIDIRECTIONAL:
> +		L2_cache_block_writeback_invalidate(paddr, paddr + size);
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	debug_dma_sync_single_for_cpu(dev, handle, size, dir);
> +}
> +EXPORT_SYMBOL(dma_sync_single_for_cpu);

Normally, sync_*_for_cpu is the same as unmap regarding the cache.

> +void dma_sync_single_for_device(struct device *dev, dma_addr_t handle,
> +				size_t size, enum dma_data_direction dir)
> +{
> +	unsigned long paddr = handle;
> +
> +	BUG_ON(!valid_dma_direction(dir));
> +
> +	/* don't bother invalidating if DMA to device */
> +	if (dir != DMA_TO_DEVICE)
> +		L2_cache_block_invalidate(paddr, paddr + size);
> +
> +	debug_dma_sync_single_for_device(dev, handle, size, dir);
> +}
> +EXPORT_SYMBOL(dma_sync_single_for_device);

And this should do the same as map(). In particular it needs
to writeback the cache when direction is TO_DEVICE or BIDIRECTIONAL.

	Arnd
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