On Wednesday 27 April 2011 23:37:51 Benjamin Herrenschmidt wrote:
On Wed, 2011-04-27 at 08:35 +0100, Russell King - ARM Linux wrote:
On Thu, Apr 21, 2011 at 09:29:16PM +0200, Arnd Bergmann wrote:
- Implement dma_alloc_noncoherent on ARM. Marek pointed out that this is needed, and it currently is not implemented, with an outdated comment explaining why it used to not be possible to do it.
dma_alloc_noncoherent is an entirely pointless API afaics.
I was about to ask what the point is ... (what is the expected semantic ? Memory that is reachable but not necessarily cache coherent ?)
Drivers use this when they explicitly want to manage the caching themselves. I think this is most interesting on big NUMA systems, where you really want to use fast (local cached) memory and then flush it explicitly to do dma. Very few drivers use this:
arnd@wuerfel:~/linux-2.6$ git grep dma_alloc_noncoherent drivers/ drivers/base/dma-mapping.c: vaddr = dma_alloc_noncoherent(dev, size, dma_handle, gfp); drivers/net/au1000_eth.c: aup->vaddr = (u32)dma_alloc_noncoherent(NULL, MAX_BUF_SIZE * drivers/net/lasi_82596.c:#define DMA_ALLOC dma_alloc_noncoherent drivers/net/sgiseeq.c: sr = dma_alloc_noncoherent(&pdev->dev, sizeof(*sp->srings), drivers/scsi/53c700.c: memory = dma_alloc_noncoherent(hostdata->dev, TOTAL_MEM_SIZE, drivers/scsi/sgiwd93.c: hdata->cpu = dma_alloc_noncoherent(&pdev->dev, HPC_DMA_SIZE, drivers/tty/serial/mpsc.c: } else if ((pi->dma_region = dma_alloc_noncoherent(pi->port.dev, drivers/video/au1200fb.c: fbdev->fb_mem = dma_alloc_noncoherent(&dev->dev,
So you can't have a dma_map_ops for the cache handling bits, a dma_map_ops for IOMMU, and a dma_map_ops for the dmabounce stuff. It just doesn't work like that.
Well, the dmabounce and cache handling is one implementation that's just on/off with parameters no ?. iommu is different implementations. So the ops should be for the iommu backends. The dmabounce & cache handling is then done by those backends based on flags you stick in struct device for example.
Well, what we are currently discussing is to have a common implementation for IOMMUs that provide the generic iommu_ops that the KVM people introduced. Once we get there, we only need a single dma_map_ops structure for all IOMMUs.
Arnd