On Wed, Jan 14, 2009 at 7:35 AM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > On Wed, Jan 14, 2009 at 8:32 AM, Vineet Gupta <vineetg76@xxxxxxxxx> wrote: >> Hi, >> >> I've read dma mapping and dma-api kernel documentation as well as the >> DMA API change at http://lwn.net/Articles/75780/ which describes fork >> of dma_sync_* to dma_sync_*_for_cpu and dma_sync_*_for_device. >> >> It seems the names of API are self explanatory and direction parameter >> is not needed (except for handling BIDIRECTIONAL case). >> So for e.g. If software wants to access the dma buffer, post DMA, it >> will invoke dma_sync_single_for_cpu so that non-coherent arch can >> invalidate the corresponding cache lines. This implies FROM_DEVICE >> semantics. Does that mean dma_sync_single_for_cpu invoked with >> direction = TO_DEVICE should return error. > > drivers/ieee1394/sbp2.c: > > static int sbp2_create_command_orb(struct sbp2_lu *lu, > struct sbp2_command_info *cmd, > struct scsi_cmnd *SCpnt) > { > dma_sync_single_for_cpu(dmadev, cmd->command_orb_dma, > sizeof(struct sbp2_command_orb), DMA_TO_DEVICE); > > ??? Yes I've seen this, but fail to understand the semantics. sync_for_cpu implies transfer of ownership to CPU, after a DMA, which means FROM_DEVICE semantics. To prepare the buffer to be CPU ready, non coherent arch needs to invalidate any stray cache lines. However passing a TO_DEVICE direction to dma_sync_single_for_cpu will do just reverse (look at ARM implementation of this function), it wbacks the cache lines. So my point is dma_sync_single_for_cpu with direction = DMA_TO_DEVICE sounds ambiguous and calling driver is not sure what kind of cache sync operation he implies. >> Corollary case is when dma_sync_single_for_device is invoked with >> direction = FROM_DEVICE, is tha error too (buggy driver). >> >> The architecture for which I'm implementing DMA coherency APIs has >> non-coherent DMA memory and logical-to-bus mapping / DMA bounce is not >> a issue. >> >> Thanks, >> Vineet >> > > > -- > Regards, > Peter Teoh > -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
