Re: mmap(2)ping of pci_alloc_consistent() allocated buffers on 2.4 kernels question/help

[Martin Drab <drab@kepler.fjfi.cvut.cz>]

On Fri, 16 Sep 2005, Martin Drab wrote:
> On Fri, 16 Sep 2005, Arjan van de Ven wrote:
> > On Fri, 2005-09-16 at 20:33 +0200, Martin Drab wrote:
> > > Hi,
> > > 
> > > can anyone explain me why it is not possible to mmap(2) a buffer 
> > > allocated in kernel by pci_alloc_consistent() to userspace on a 2.4 
> > > kernel?
> > > 
> > > In kernel PCI device initialization function I do something like:
> > > 
> > > ...
> > > kladdr = pci_alloc_consistent (dev, BUFSIZE, &baddr);
> > > ...
> > > 
> > > Then I send the physical address (i.e. the value of phaddr = __pa(kladdr)) 
> > > to the userspace application, and then when in the userspace I do 
> > > something like
> > > 
> > > ...
> > > fd = fopen ("/dev/mem", O_RDWR);
> > > buf = mmap (NULL, BUFSIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, phaddr);
> > > ...
> > 
> > yuch.
> > why don't you make your device have an mmap operation instead?
> > (the device node that you use to get your physical address to userspace
> > in the first place)
> 
> And would that help anyhow?
... 
> 
> There's just a limited way to communicate between the RT and non-RT part 
> of the kernel/app. The RT-FIFOs and their IOCTLs are safe. I'm not 
> entirely sure a simple mmap would be safe here, but I may try that.

Well, now that I think about it, I think it may be done in a "safe" way, 
since the RT would be in total controll over the buffers, and would 
allow to mmap only the filled buffers after they are filled. But that 
would assume, that the app. behaves politely and only asks for the buffers 
that it gets a notification for. Or am I mistaken somewhere?

Martin
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