DMA to/from user buffers

DMA to/from user buffers

[Sebastian Heidl]

Hi,

two questions about using an user-supplied buffer (e.g. malloced
in user space) for DMA transfers:

1. Is it possible ?
2. What restrictions/requirements apply for the buffer (alignment...) ?

Documentation/DMA-mapping.txt only talks about buffers allocated in
kernel space.

thanks,
_sh_

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Re: DMA to/from user buffers

[Gerd Knorr]

Sebastian Heidl wrote:
>  
>  Hi,
>  
>  two questions about using an user-supplied buffer (e.g. malloced
>  in user space) for DMA transfers:
>  
>  1. Is it possible ?
>  2. What restrictions/requirements apply for the buffer (alignment...) ?
>  
>  Documentation/DMA-mapping.txt only talks about buffers allocated in
>  kernel space.

Lock down the user pages using kiobufs (see include/linux/iobufs.h),
then feed these pages to the pci_* functions.

  Gerd

-- 
Damn lot people confuse usability and eye-candy.

Re: DMA to/from user buffers

[Helge Hafting]

Sebastian Heidl wrote:
> 
> Hi,
> 
> two questions about using an user-supplied buffer (e.g. malloced
> in user space) for DMA transfers:
> 
> 1. Is it possible ?
> 2. What restrictions/requirements apply for the buffer (alignment...) ?

There are some obvious problems with this.  First, a malloc'ed
buffer might not be in memory.  It might not be created yet if you
haven't touched it, and it might be on swap if you have.  

The recommended way is to allocate in the kernel, (probably by
the driver that want to do DMA), and mmap this buffer into
userspace.  

Helge Hafting