On Wed, Jun 11, 2014 at 12:23 PM, Bob Biloxi <iambobbiloxi@xxxxxxxxx> wrote: > Hi All, > > I am having trouble understanding DMA window and actual amount of > addressable DMA memory. > > I hope someone explains me. Let me put my understanding and doubts here: > > Let's say I am writing code for an ethernet device driver in the > virtualisation(hypervisor) environment. > > Now, if the ethernet adapter requires certain amount of DMA memory, I > need to allocate heap memory and dma map it and provide to the > adapter. > > From the hardware perspective, we have a 64GB DMA window. > > I am having trouble understanding this value. Does it mean i can > allocate 64GB of RAM(Heap memory) and dma map it? > > As i understand there might be a table that translates bus address to > physical(RAM) addresses. Each entry of such table points to a 4KB > page. If the size of each entry is 8 bytes and there are 16M such > entries( 16M * 4K = 64GB DMA window), the size of the table comes to > around 128M > > Now do I have 64GB DMA memory or 128M DMA memory? Documentation/DMA-API-HOWTO.txt might help answer your questions. The bus address to RAM address translation is done by a IOMMU hardware. The tables you mention are I/O page tables used by the IOMMU. The 128M occupied by the tables is kernel bookkeeping overhead and has nothing to do with the adapter itself. The 64GB DMA window might be a hardware feature of the device, i.e., maybe it can only generate 36-bit DMA addresses. That doesn't mean you have to allocate memory for the whole window; I would guess drivers would only allocate and map what they need. I don't know how they figure out how much to map. > I want to know what is the max amount of memory that I can > allocate(heap), dma map and provide it to the adapter. > > I will be really thankful in all the help that I can get!! > > Thanks so much > > > Best Regards, > Marc > -- > To unsubscribe from this list: send the line "unsubscribe linux-pci" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
