Re: [PATCH 0/7] DMAENGINE: fixes and PrimeCells

[Dan Williams]

On Fri, May 7, 2010 at 7:37 PM, jassi brar <jassisinghbrar@xxxxxxxxx> wrote:
> On Sat, May 8, 2010 at 1:10 AM, Linus Walleij
> <linus.ml.walleij@xxxxxxxxx> wrote:
>> Surely circular linked buffers and other goodies can be retrofitted into the
>> DMAengine without a complete redesign? I only see a new slave call
>> to support that really, in addition to the existing sglist interface.
> well, before taking up the PL330 dma api driver, 'async' character of it
> was the only concern I had in mind. That still is, but I came across a
> a few more peculiarities while implementing the driver.
>
> a) Async:- For lazy transfers of mem to mem this may be ok.
>  But there might be devices the employ DMA to do extensive M2M transfers
>  (say dedicated multimedia oriented devices) the 'async' nature might be
>  a bottleneck. So too for M<=>D with a fast device with shallow FIFO.
>  There may be clients that don't wanna do much upon DMA done, but they
>  do need notifications ASAP.  By definition, this API forbids such
> expectations.

It is not forbidden by definition.  What is needed is a way for
drivers to opt-out of the async_tx expectations.  I have started down
this path with CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH for the ioatdma
driver, but the idea could be extended further to disable
CONFIG_ASYNC_TX_DMA and NET_DMA entirely to allow the device to
operate in a more device-dma friendly mode.

>  IMHO, a DMA api should be as quick as possible - callbacks done in IRQ context.
>  But since there maybe clients that need to do sleepable stuff in

None of the current clients sleep in the callback, it's done in
soft-irq context.  The only expectation is that hard-irqs are enabled
during the callback just like timer callbacks.  I also would like to
see numbers to quantify the claims of slowness.  When Steven Rostedt
was proposing his "move tasklets to process context" patches I ran a
throughput test on iop13xx and did not measure any degradation.

> callbacks, the API
>  may do two callbacks - 'quick' in irq context and 'lazy' from
> tasklets scheduled from
>  the IRQ. Most clients will provide either, while some may provide
> both callback functions.
>
> b) There seems to be no clear way of reporting failed transfers. The
> device_tx_status
>    can get FAIL/SUCSESS but the call is open ended and can be performed
>    without any time bound after tx_submit. It is not very optimal for
> DMAC drivers
>    to save descriptors of all failed transactions until the channel
> is released.
>    IMHO, provision of status checking by two mechanisms: cookie and dma-done
>   callbacks is complication more than a feature. Perhaps the dma
> engine could provide
>   a default callback, should the client doesn't do so, and track
> done/pending xfers
>  for such requests?

I agree the error handling was designed around mem-to-mem assumptions
where failures are due to double-bit ECC errors and other rare events.

>
> c) Conceptually, the channels are tightly coupled with the DMACs,
> there seems to be
>   no way to be able to schedule a channel among more than one DMACs
> in the runtime,
>   that is if more then one DMAC support the same channel/peripheral.
>   For example, Samsung's S5Pxxxx have many channels available on more
> than 1 DMAC
>   but for this dma api we have to statically assign channels to
> DMACs, which may result in
>   a channel acquire request rejected just because the DMAC we chose
> for it is already
>   fully busy while another DMAC, which also supports the channel, is idling.
>   Unless we treat the same peripheral as, say, I2STX_viaDMAC1 and
> I2STX_viaDMAC2
>   and allocate double resources for these "mutually exclusive" channels.

I am not understanding this example.  If both DMACs are registered the
dma_filter function to dma_request_channel() can select between them,
right?

>
> d) Something like circular-linked-request is highly desirable for one
> of the important DMA
>   clients i.e, audio.

Is this a standing dma chain that periodically a client will say "go"
to re-run those operations?  Please enlighten me, I've never played
with audio drivers.

>
> e) There seems to be no ScatterGather support for Mem to Mem transfers.

There has never been a use case, what did you have in mind.  If
multiple prep_memcpy commands is too inefficient we could always add
another operation.

> Or these are just due to my cursory understanding of the DMA Engine core?...

No, it's a good review and points out some places where the API can evolve.

--
Dan
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