[Question] devm_kmalloc() for DMA ?

[Russell King - ARM Linux]

On Wed, Mar 08, 2017 at 08:48:31PM +0100, Lars-Peter Clausen wrote:
> When the DMA memory is mapped for reading from the device the associated
> cachelines are invalidated without writeback. There is no guarantee that
> the changes made to the devres_node have made it to main memory yet, or
> is there?

That is incorrect.

Overlapping cache lines are always written back on transitions from CPU
to device ownership of the buffer (eg, dma_map_*().)

Updates that are made by the CPU on overlapping cache lines while the
memory is mapped for DMA may end up doing one of two things: either
overwriting the newly DMA'd data, or being lost altogether.

In the case of devm_* list manipulations, these should only ever happen
during device probe and tear down, and if DMA is active at those times,
the driver is seriously buggy.

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