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Diffstat (limited to 'Documentation/block/writeback_cache_control.rst')
| -rw-r--r-- | Documentation/block/writeback_cache_control.rst | 67 |
1 files changed, 38 insertions, 29 deletions
diff --git a/Documentation/block/writeback_cache_control.rst b/Documentation/block/writeback_cache_control.rst index b208488d0aae..c3707d071780 100644 --- a/Documentation/block/writeback_cache_control.rst +++ b/Documentation/block/writeback_cache_control.rst @@ -46,41 +46,50 @@ worry if the underlying devices need any explicit cache flushing and how the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags may both be set on a single bio. +Feature settings for block drivers +---------------------------------- -Implementation details for bio based block drivers --------------------------------------------------------------- +For devices that do not support volatile write caches there is no driver +support required, the block layer completes empty REQ_PREFLUSH requests before +entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from +requests that have a payload. -These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit -directly below the submit_bio interface. For remapping drivers the REQ_FUA -bits need to be propagated to underlying devices, and a global flush needs -to be implemented for bios with the REQ_PREFLUSH bit set. For real device -drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits -on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without -data can be completed successfully without doing any work. Drivers for -devices with volatile caches need to implement the support for these -flags themselves without any help from the block layer. +For devices with volatile write caches the driver needs to tell the block layer +that it supports flushing caches by setting the + BLK_FEAT_WRITE_CACHE -Implementation details for request_fn based block drivers ---------------------------------------------------------- +flag in the queue_limits feature field. For devices that also support the FUA +bit the block layer needs to be told to pass on the REQ_FUA bit by also setting +the -For devices that do not support volatile write caches there is no driver -support required, the block layer completes empty REQ_PREFLUSH requests before -entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from -requests that have a payload. For devices with volatile write caches the -driver needs to tell the block layer that it supports flushing caches by -doing:: + BLK_FEAT_FUA + +flag in the features field of the queue_limits structure. + +Implementation details for bio based block drivers +-------------------------------------------------- + +For bio based drivers the REQ_PREFLUSH and REQ_FUA bit are simply passed on to +the driver if the driver sets the BLK_FEAT_WRITE_CACHE flag and the driver +needs to handle them. + +*NOTE*: The REQ_FUA bit also gets passed on when the BLK_FEAT_FUA flags is +_not_ set. Any bio based driver that sets BLK_FEAT_WRITE_CACHE also needs to +handle REQ_FUA. - blk_queue_write_cache(sdkp->disk->queue, true, false); +For remapping drivers the REQ_FUA bits need to be propagated to underlying +devices, and a global flush needs to be implemented for bios with the +REQ_PREFLUSH bit set. -and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that -REQ_PREFLUSH requests with a payload are automatically turned into a sequence -of an empty REQ_OP_FLUSH request followed by the actual write by the block -layer. For devices that also support the FUA bit the block layer needs -to be told to pass through the REQ_FUA bit using:: +Implementation details for blk-mq drivers +----------------------------------------- - blk_queue_write_cache(sdkp->disk->queue, true, true); +When the BLK_FEAT_WRITE_CACHE flag is set, REQ_OP_WRITE | REQ_PREFLUSH requests +with a payload are automatically turned into a sequence of a REQ_OP_FLUSH +request followed by the actual write by the block layer. -and the driver must handle write requests that have the REQ_FUA bit set -in prep_fn/request_fn. If the FUA bit is not natively supported the block -layer turns it into an empty REQ_OP_FLUSH request after the actual write. +When the BLK_FEAT_FUA flags is set, the REQ_FUA bit is simply passed on for the +REQ_OP_WRITE request, else a REQ_OP_FLUSH request is sent by the block layer +after the completion of the write request for bio submissions with the REQ_FUA +bit set. |