Re: correct use of bus_dmamap_sync

From: John Baldwin (jhb_at_freebsd.org)
Date: 10/25/05

  • Next message: Ruslan Ermilov: "Re: correct use of bus_dmamap_sync" 
    To: Dinesh Nair <dinesh@alphaque.com>
Date: Tue, 25 Oct 2005 16:10:52 -0400

    On Tuesday 25 October 2005 02:46 pm, Dinesh Nair wrote:
    > On 10/26/05 01:27 John Baldwin said the following:
    > > On Tuesday 25 October 2005 09:15 am, Dinesh Nair wrote:
    > >>(must i malloc space for them before passing them into those functions,
    > >> or will the call to bus_dmamem_alloc do it for me ?)
    > >
    > > bus_dmamem_alloc() will do it for you.
    >
    > thanx.
    >
    > > Probably not as the sync()'s don't really do anything with memory
    > > allocated via bus_dmamem_alloc(). The operations are named from the
    > > CPU's perspective,
    >
    > however, the man page at
    > http://www.freebsd.org/cgi/man.cgi?query=bus_dmamap_sync&apropos=0&sektion=
    >0&manpath=FreeBSD+5.4-stable&format=html says,
    >
    > "Although no explicit loading is required to access the memory referenced
    > by the returned map, the synchronization requirements as described in the
    > bus_dmamap_sync() section still apply."

    Yes, and on some archs the sync() operations do have memory barriers in place,
    but there isn't any bounce buffering with bus_dmamem_alloc() memory.

    > also, is bus_dmamap_load() required, since the same man page section above
    > says it isnt ?

    Well, you need it to get the physical address to pass to your device for it to
    do DMA against.

    > have things changed between freebsd 4.x (which i'm using) and freebsd 5.x ?

    I don't think so as far as the interface.

    > > thus when you send data to your device, that is a WRITE operation (even
    > > though your device is doing a DMA to read data), and when you get data
    > > back from your device, that is a READ operation (even though your device
    > > is doing a DMA to write the data into the buffer).
    >
    > thanx, the verbiage on the man page is slightly confusing with it's use of
    > CPU, giving the opposite impression.

    Yes, I know. :) 

    -- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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  • Next message: Ruslan Ermilov: "Re: correct use of bus_dmamap_sync"

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