linux-kernel - Re: [PCI Driver]Physical address being returned as 0 in mmap

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Message-ID: <BANLkTikO+cUbCFPAuNSE01xSmOzhwWP0mQ@mail.gmail.com>
Date:	Sat, 2 Jul 2011 17:27:20 +0200
From:	newton mailinglist <newtonmailinglist@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: [PCI Driver]Physical address being returned as 0 in mmap

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@...fetthome.net> wrote:
>> On Fri, Jul 1, 2011 at 20:05, newton mailinglist
>> <newtonmailinglist@...il.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
>>
>
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