On Thu, Mar 10, 2016 at 8:05 AM, Niklas S??derlund <niklas.soderlund@xxxxxxxxxxxx> wrote: > Hi Christoph, > > On 2016-03-07 23:38:47 -0800, Christoph Hellwig wrote: >> Please add some documentation on where/how this should be used. It's >> not a very obvious interface. > > Good idea, I have added the following to Documentation/DMA-API.txt and > folded it in to this patch. Do you feel it's adequate and do you know > anywhere else I should add documentation? > > diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt > index 45ef3f2..248556a 100644 > --- a/Documentation/DMA-API.txt > +++ b/Documentation/DMA-API.txt > @@ -277,14 +277,29 @@ and <size> parameters are provided to do partial page mapping, it is > recommended that you never use these unless you really know what the > cache width is. > > +dma_addr_t > +dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size, > + enum dma_data_direction dir, struct dma_attrs *attrs) > + > +Maps a MMIO region so it can be accessed by the device and returns the > +DMA address of the memory. API should only be used to map device MMIO, > +mapping of RAM is not permitted. > + I think it is confusing to use the dma_ prefix for this peer-to-peer mmio functionality. dma_addr_t is a device's view of host memory. Something like bus_addr_t bus_map_resource(). Doesn't this routine also need the source device in addition to the target device? The resource address is from the perspective of the host cpu, it may be a different address space in the view of two devices relative to each other.
