RE: DMA memory limitation?

[Richard B. Johnson]

On Thu, 5 Jul 2001, Christophe Beaumont wrote:

> 
> 
> > On Thu, 5 Jul 2001, Vasu Varma P V wrote:
> > 
> > > Hi,
> > > 
> > > Is there any limitation on DMA memory we can allocate using
> > > kmalloc(size, GFP_DMA)? I am not able to acquire more than
> > > 14MB of the mem using this on my PCI SMP box with 256MB ram.
> > > I think there is restriction on ISA boards of 16MB.
> > > Can we increase it ?
> > > 
> > > thx,
> > > Vasu
> > 
> > 14MB of DMA(able) memory?  Err. I think you are trying to
> > do something you would never need to do.
> > 
> And what is that supposed to be????
> 

Did you READ the rest of the carefully-worded paragraphs,
carefully, explaining that it ** ISN'T DMA ** memory that you need?

> I have a piece of pretty well designed hardware here... and my
> application requires me to have the PCI board to random access
> in master mode a whole lot of memory (anywhere from 128MEGS to
     ^^^^^^^^^^^
You JUST give it the address of any RAM you want to read/write...
as explained before. As explained, in a documentation file I
specified, you convert the virtual address using a macro that
the kernel headers provide.

You can even give it user-mode buffers as long as they are non-paged
during the access.

> 1GIG... and possibly more...) so I really do need BIG DMA buffers
> (I don't say huge anymore as one can get 1/2 Gig of RAM for just
> over 120 bucks???)... 
> 
> There is no way I can have the piece of hardware behave in 
> another fashion... and NO it is NOT broken (when you do BOTH 
> hardware & software, you know about BOTH limitations... there
> are just some cases where you have to face some unique issues).
> the mem=1024M works pretty fine once you have figured out how 
> to handle in a fairly simple way this "reserved" memory...
> 

This is wrong! Good luck sucker.

> So please software people...
                     ^^^^^^^

I am a Software Engineer, with a long history of hardware design.

Cheers,
Dick Johnson

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