Tearing down DMA transfer setup after DMA client has finished

[Måns Rullgård]

Mason <slash.tmp at free.fr> writes:

> On 06/12/2016 06:12, Vinod Koul wrote:
>
>> On Tue, Nov 29, 2016 at 07:25:02PM +0100, Mason wrote:
>> 
>>> Is there a way to write a driver within the existing framework?
>> 
>> I think so, looking back at comments from Russell, I do tend to agree with
>> that. Is there a specific reason why sbox can't be tied to alloc and free
>> channels?
>
> Here's a recap of the situation.
>
> The "SBOX+MBUS" HW is used in several iterations of the tango SoC:
>
> tango3
>   2 memory channels available
>   6 devices ("clients"?) may request an MBUS channel
>
> tango4 (one more channel)
>   3 memory channels available
>   7 devices may request an MBUS channel :
>     NFC0, NFC1, SATA0, SATA1, memcpy, (IDE0, IDE1)
>
> Notes:
> The current NFC driver supports only one controller.

I consider that a bug.

> IDE is mostly obsolete at this point.
>
> tango5 (SATA gets own dedicated MBUS channel pair)
>   3 memory channels available
>   5 devices may request an MBUS channel :
>     NFC0, NFC1, memcpy, (IDE0, IDE1)

Some of the chip variants can also use this DMA engine for PCI devices.

> If I understand the current DMA driver (written by Mans), client
> drivers are instructed to use a specific channel in the DT, and
> the DMA driver muxes access to that channel.

Almost.  The DT indicates the sbox ID of each device.  The driver
multiplexes requests from all devices across all channels.

> The DMA driver manages a per-channel queue of outstanding DMA transfer
> requests, and a new transfer is started friom within the DMA ISR
> (modulo the fact that the interrupt does not signal completion of the
> transfer, as explained else-thread).

We need to somehow let the device driver signal the dma driver when a
transfer has been fully completed.  Currently the only post-transfer
interaction between the dma engine and the device driver is through the
descriptor callback, which is not suitable for this purpose.

This is starting to look like one of those situations where someone just
needs to implement a solution, or we'll be forever bickering about
hypotheticals.

> What you're proposing, Vinod, is to make a channel exclusive
> to a driver, as long as the driver has not explicitly released
> the channel, via dma_release_channel(), right?

That's not going to work very well.  Device drivers typically request
dma channels in their probe functions or when the device is opened.
This means that reserving one of the few channels there will inevitably
make some other device fail to operate.

Doing a request/release per transfer really doesn't fit with the
intended usage of the dmaengine api.  For starters, what should a driver
do if all the channels are currently busy?

Since the hardware actually does support multiplexing the dma channels,
I think it would be misguided to deliberately cripple the software
support in order to shoehorn it into an incomplete model of how hardware
ought to work.  While I agree it would be nicer if all hardware actually
did work that way, this isn't the reality we're living in.

-- 
Måns Rullgård