Re: Address spaces on a i386 - Getting Confused (fwd)

Ramakrishna K (ramak@wipinfo.soft.net)
Thu, 1 Apr 1999 08:36:39 +0530 (IST)

> Ramakrishna K wrote:
> > I am not familiar with Linux code.
> >
> > But a solution to DMA'ing to user-space will be
> > a) lock the relevant user range into memory.
> > b) touch them to fault the pages in.
> > c) Then trap into the kernel using an ioctl to set up kernel address for this
> > range. ( I bet there must be some VM calls to acheive that ). What i mean
> > is that get the pages behind the locked user-address behind the kernel address.
> > d) Do the DMA to these pages.
> > e) Release the kernel-mappings.
>
> You make it sound so simple. Unfortunately it isn't.
>
> c) as you describe not possible: In the kernel, a "kernel address" that
> can be DMA'd using the documented method (virt_to_bus) is in the direct
> mapped region. On i386, that's the region from 0xc0000000 up.
>
> You can set up a kernel-space mapping (similar to vmalloc), however DMA
> using virt_to_bus doesn't work for them.
This should not be a big effort to change for user space if it's not already available.

But i do agree that it's going to be a future problem for cards which do not see
memory range above 4Gb.

> The device driver must be written to divide its DMA requirements into
> regions that don't cross non-contiguous page boundaries. It must look
But most of the cards do support scatter gather right ?

> up the physical address from user-space (an architecture specific thing
> for which there is no reliable macro yet). It must translate the
> physical address to a bus address. (Possible using
> virt_to_bus(phys_to_virt()) for the moment).

thanks,
Rama.

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