Virtual To Physical Address

Is virt_to_bus() not giving you what you need ?

Don't forget to pass a virtual address that's suitable for DMA operations. On x86, I use __get_free_page() for this purpose.

You can find the LDD book online at:

Simon.

Thanks Simon, I have used one function pci_alloc_consistent() which does the the same functionality what u have said above. But I didn't understand that DMA needs virtual address that's suitable for DMA operation, please give me detail's abt it. Here I am using Mips processor.

Prakash.

Some devices require that the DMA buffers be aligned on some boundary (for example page aligned). For example, UHCI controllers require that the frame page be aligned on a 4096 byte boundary, and the transfer descriptors/queue headers to be aligned on 16 byte boundaries.

If the device doing DMA needs to transfer more than one page of memory at a time, it can also require that the pages be physically contiguous instead of just virtually contiguous. That requires operating system support, (in the case of Linux that's by using routines such as __get_free_pages()) and means that you cannot, for example, just define a large array within your driver and take the physical address of the array's base.

Finally, assuming that you have taken care of alignment and any possible contiguous memory issues, you still need to pass an address that will not be paged out by the kernel while the DMA is in progress. (You can't, for example, DMA directly into a user space buffer unless you have locked down the buffer.)

The pci_alloc_consistent()/__get_free_page[s]() routines handle all this for you.

Simon.