mmc: Add MMC host software queue support
Now the MMC read/write stack will always wait for previous request is completed by mmc_blk_rw_wait(), before sending a new request to hardware, or queue a work to complete request, that will bring context switching overhead and spend some extra time to poll the card for busy completion for I/O writes via sending CMD13, especially for high I/O per second rates, to affect the IO performance. Thus this patch introduces MMC software queue interface based on the hardware command queue engine's interfaces, which is similar with the hardware command queue engine's idea, that can remove the context switching. Moreover we set the default queue depth as 64 for software queue, which allows more requests to be prepared, merged and inserted into IO scheduler to improve performance, but we only allow 2 requests in flight, that is enough to let the irq handler always trigger the next request without a context switch, as well as avoiding a long latency. Moreover the host controller should support HW busy detection for I/O operations when enabling the host software queue. That means, the host controller must not complete a data transfer request, until after the card stops signals busy. From the fio testing data in cover letter, we can see the software queue can improve some performance with 4K block size, increasing about 16% for random read, increasing about 90% for random write, though no obvious improvement for sequential read and write. Moreover we can expand the software queue interface to support MMC packed request or packed command in future. Reviewed-by:Arnd Bergmann <arnd@arndb.de> Signed-off-by:
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