Messages in this thread | | | Date | Sat, 2 Jul 2011 17:27:20 +0200 | Subject | Re: [PCI Driver]Physical address being returned as 0 in mmap | From | newton mailinglist <> |
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ok thanks Kyle, I will try the DMA API now.
I was wondering, when my driver gets the mmap() call then has the kernel already allocated memory and put its details in the vma parameter or do I need to allocate memory myself ?
I am already using the driver read and write functions to read/write some other parameters to the FPGA, so I will setup the DMA in mmap()
Cheers Abhi
> On Sat, Jul 2, 2011 at 3:54 AM, Kyle Moffett <kyle@moffetthome.net> wrote: >> On Fri, Jul 1, 2011 at 20:05, newton mailinglist >> <newtonmailinglist@gmail.com> wrote: >>> I have written a PCI driver for my device. The device is an FPGA which >>> is configured with a design that allows it to have direct access to >>> memory of a host computer to which the fpga board is connected. My >>> device driver is responsible for translating virtual addresses to >>> physical addresses and sending these to the FPGA so the DMA unit in >>> the FPGA can directly access pages in memory using the physical >>> address. >>> >>> [...snip...] >>> >>> When the driver gets the above call then I assume the kernel has >>> already allocated the required space and its details are in the vma >>> parameter. When I try to view the physical address of the virtual >>> address in the vma i use : >>> >>> virt_to_phys(vma->vm_start) >> >> You should not use virt_to_phys(). On most systems it does not work >> for all memory, and on some (with an IOMMU) it would need to give a >> different result depending on the device. >> >> What you are looking for is the DMA API: >> >> http://lxr.linux.no/#linux/Documentation/DMA-API-HOWTO.txt >> http://lxr.linux.no/#linux/Documentation/DMA-API.txt >> >> Specifically, you need to specify the DMA address mask of your FPGA >> since not all devices can access 100% of the address space, some >> are limited to 24/31/32/40-bit address spaces. See dma_set_mask(). >> >> If you don't need fully coherent memory (as described in those docs), >> then you can simply allow read() and write() on your device node and >> perform DMA mapping of the userspace memory using dma_map_*(). >> >> Otherwise you will mmap() the device and use dma_alloc_coherent() >> or dma_pool_*(), this is typical for things like TX queues and such. >> >> Then make sure to insert calls to dma_sync_*() when switching >> between CPU access to memory and device access to memory, >> and ensure that you use appropriate IO memory barriers to order >> your register reads and writes. >> >> If you don't follow the DMA API specifications then your driver will >> mysteriously fail on many platforms, even x86 if an IOMMU is present, >> but particularly on processors like ARM with poor cache coherency. >> >> Cheers, >> Kyle Moffett >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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