On 11/26/2014 09:26 PM, Julian Scheel wrote:
Hi, I stumbled into an issues with the tegra-serial driver, which fails to acquire a dma channel after the first shutdown() call on any of the channels. The effect is, that once a ttyTHSx device was opened and then closed none of the existang ttyTHSx devices can be opened anymore. I tracked this down to some messing with the DMA_PRIVATE flag. The tegra-serial driver makes use of dma_request_slave_channel_reason to acquire a tx and a rx channel. Via of_dma_request_slave_channel the call goes through tegra_dma_of_xlate which finally calls dma_get_any_slave_channel. This again calls private_candidate to actually select a channel and finally dma_chan_get to acquire the channel. On shutdown the tegra-serial driver calls dma_release_channel for both channels. Now this is where things break, because dma_release_channel does decrease privatecnt and drops the DMA_PRIVATE flag once the privatecnt reached zero. This happens immediately, so that the flag is cleared and the next call to private_candidate will fail. The comment before decreasing privatecnt suggests that it is assumed that the channel which is release was acquired by __dma_request_channel, which increases the privatecnt. This allows two conclusion: (a) dma_request_slave_channel and dma_request_slave_channel_reason shall increase the privatecnt just like __dma_request_channel does. (b) dma_release_channel must not be used for channels which have not been acquired via __dma_request_channel. I have implemented (a) in my tree and it works, but as I am copletely new to dmaengine I am very unsure whether this is the right approach.
dma_get_any_slave_channel() should behave the same as __dma_request_channel() in this regard. Best is to probably factor it out into a helper function dma_get_private_chan() or similar that can be used by both of the functions.
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