Re: __get_free_pages(): is the MEM really mine?

[Bernd Harries <bha@gmx.de>]

"Eric W. Biederman" wrote:

> Ouch.  This is where I give you the standard recommendation.  If you
> do this scatter gatter (so you don't need megs of continuous memory)
> you should be much better off, and your driver should be more
> reliable.

Yep, the firmware on the pixel DSP behind the PLX-9054 bridge wants
the base address of a linear 2K * 1K * 2 picture buffer so that it can
trigger the one of the 9054's DMA engines to transfer triangles line 
by line into my memory buffer. If I mmap the PCI space to userland, each read
cycle costs 700-900 ns. The DMA engine can use bursts and then a cycle costs
only 29.9 ns of PCI bandwidth.

>  All of the other techniques you have used like mmap should
> still apply.

> Also if you are exporting this data to user space, before your DMA
> complets you want to zero the pages you have allocated, so you don't
> have an information leak.

The DMA engine in the PLX 9054 can at least do Write-and-Invalidate cycles to
the Main RAM. :-)

Ciao,
-- 
Bernd Harries

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