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    SubjectRe: [PATCHv2] DMAEngine: Let dmac drivers to set chan_id
    On Wed, Jul 27, 2011 at 08:00:23PM +0530, Jaswinder Singh wrote:
    > On 27 July 2011 14:32, Koul, Vinod <> wrote:
    > > You can have two different DMACs in same system. At least I have two
    > > from current intel_mid_dma which are used. Both give their channel id
    > > starting from 0, 1....
    > > Further as we integrate video, audio, spi, emmc dmacs possibility of
    > > having multiple dmacs will increase in a system
    > Most of Samsung's S5P series have 3 DMACs - 2 for peripherals and 1 for
    > mem->mem But that is not the point.
    > This patch in no way affects what values currently a dmac driver
    > assigns to chan_id

    Then *explain* how the chan_id is used to match the channel which the
    peripheral requires when you have three DMA controllers, each with
    channels numbered 0 to 7.

    > > Sorry I didn't get you.
    > > As I understand you are trying to simplify the filter function by
    > > assigning unique ids to all channels,
    > No dear. Let me put it precisely.
    > Even if we make no further change to the dmaengine, this patch is the right
    > thing to do today.

    You sound like a politician. "the right thing to do" is a cop-out. That
    says "believe me, I know I'm right, but I can't say why I'm right, I just
    am." Basically, it means that the person saying it has no clue on the
    subject they're talking about.

    If you do have a clue, then don't say that infuriating phrase, but give an
    actual reason.

    > On a serious note, my proposal, and the reply, shows the possibility
    > of having :-
    > a) Client drivers that are truly platform agnostic -- no platform_data
    > poking for
    > channel selection

    I really doubt that's even possible. Take this setup:

    MMCI ---> DMAC

    where the DMAC has 32 request signals, and 8 channels. The MMCI is
    connected to two of them. The DMAC can supply any of its physical
    channels for MMCI.

    Board 1 has the MMCI connected to request signals #1 and #3. Board 2
    has the MMCI connected to request signals #8 and #22. Board 3 has
    the MMCI connected through an external FPGA mux, which can route the
    MMCI requests to DMA request signals #1, #2 or #3.

    Now, explain how a channel is selected for each of those two boards, and
    the DMA controller is provided with the relevant request signal is found
    without platform data involved, using _only_ your 'capabilities' bitfield
    and the channel ID number.

    Russell King
    Linux kernel 2.6 ARM Linux -
    maintainer of:

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