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    SubjectRe: [PATCH 03/10] block: add rq->resid_len
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    On Wed, 2009-04-29 at 18:13 +0900, Tejun Heo wrote:
    > rq->data_len served two purposes - the length of data buffer on issue
    > and the residual count on completion. This duality creates some
    > headaches.
    >
    > First of all, block layer and low level drivers can't really determine
    > what rq->data_len contains while a request is executing. It could be
    > the total request length or it coulde be anything else one of the
    > lower layers is using to keep track of residual count. This
    > complicates things because blk_rq_bytes() and thus
    > [__]blk_end_request_all() relies on rq->data_len for PC commands.
    > Drivers which want to report residual count should first cache the
    > total request length, update rq->data_len and then complete the
    > request with the cached data length.
    >
    > Secondly, it makes requests default to reporting full residual count,
    > ie. reporting that no data transfer occurred. The residual count is
    > an exception not the norm; however, the driver should clear
    > rq->data_len to zero to signify the normal cases while leaving it
    > alone means no data transfer occurred at all. This reverse default
    > behavior complicates code unnecessarily and renders block PC on some
    > drivers (ide-tape/floppy) unuseable.
    >
    > This patch adds rq->resid_len which is used only for residual count.
    >
    > While at it, remove now unnecessasry blk_rq_bytes() caching in
    > ide_pc_intr() as rq->data_len is not changed anymore.
    >
    > [ Impact: cleanup residual count handling, report 0 resid by default ]

    This looks good (although I'd like to test it first).

    Might it not be better to have an accessor setting resid_len? All the
    other patches in the series insulate users from the actual members of
    struct request by accessors, so this is a bit the odd man out.

    James




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