Re: [PATCH v7 2/4] dmaengine: Forward slave device pointer to of_xlate callback

From: Marek Szyprowski
Date: Thu Jan 26 2017 - 09:43:34 EST

Hi Lars,

On 2017-01-25 14:12, Lars-Peter Clausen wrote:
On 01/25/2017 11:28 AM, Marek Szyprowski wrote:
Add pointer to slave device to of_dma_xlate to let DMA engine driver
to know which slave device is using given DMA channel. This will be
later used to implement non-irq-safe runtime PM for DMA engine driver.
of_dma_xlate() is used to translate from a OF phandle and a specifier to a
DMA channel. On one hand this does not necessarily mean that the channel is
actually going to be used by the slave that called the xlate function.
Modifying the driver state when a lookup of the channel is done is a
layering violation. And this approach is also missing a way to disassociate
a slave from a DMA channel, e.g. when it is no longer used.

On the other hand there are other mechanisms to translate between some kind
of firmware handle to a DMA channel which are completely ignored here.

So this approach does not work. This is something that needs to be done at
the dmaengine level, not a the firmware resource translation level. And it
needs a matching method that is called when the channel is disassociated
from a device, when the device no longer uses the DMA channel.

Frankly I agree that of_dma_xlate() should only return the requested channel
to the dmaengine core and do not do any modification in the the driver state.

However the current dma engine design and implementation breaks this rule.
Please check the drivers - how do they implement of_xlate callback. They
usually call dma_get_any_slave_channel, dma_get_slave_channel or
__dma_request_channel there, which in turn calls dma_chan_get, which then
calls back to device_alloc_chan_resources callback. Some of the drivers also
do a hardware configuration or other resource allocation in of_xlate.
This is a bit messy design and leave no place in the core to set slave device
before device_alloc_chan_resources callback, where one would expect to have
it already set.

The best place to add new calls to the dmaengine drivers to set slave device
would be just before device_alloc_chan_resources(), what in turn means that
the current dmaengine core should do in dma_chan_get(). This would require to
forward the slave device pointer via even more layers including the of_xlate
callback too. IMHO this is not worth the effort.

DMA engine core and API definitely needs some cleanup. During such cleanup
the slave device pointer might be moved out of xlate into separate callback
when the core gets ready for such operation.

I ignored other paths for other firmware handle to a DMA channel translation
mechanism because for the current pl330 driver they are simply not used. I
assume that if one needs to implement similar things for drivers relying on
them, he will update the respective DMA engine core parts.

Slave device assignments can be cleared in device_chan_release_resources if
this is needed and that what existing DMA engine drivers do with the resources
allocated in the of_xlate callback...

Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland