Re: [PATCH 4/4] block: add large command support

[FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>]

On Mon, 14 Apr 2008 13:29:20 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

> On Mon, Apr 14 2008, FUJITA Tomonori wrote:
> > This patch changes rq->cmd from the static array to a pointer to
> > support large commands.
> > 
> > We rarely handle large commands. So for optimization, a struct request
> > still has a static array for a command. rq_init sets rq->cmd pointer
> > to the static array.
> > 
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> > Cc: Jens Axboe <jens.axboe@oracle.com>
> > ---
> >  block/blk-core.c       |    1 +
> >  drivers/ide/ide-io.c   |    1 +
> >  include/linux/blkdev.h |   12 ++++++++++--
> >  3 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 6669238..6f0968f 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -132,6 +132,7 @@ void rq_init(struct request_queue *q, struct request *rq)
> >  	rq->errors = 0;
> >  	rq->ref_count = 1;
> >  	rq->cmd_len = 0;
> > +	rq->cmd = rq->__cmd;
> >  	memset(rq->cmd, 0, BLK_MAX_CDB);
> >  	rq->data_len = 0;
> >  	rq->extra_len = 0;
> > diff --git a/drivers/ide/ide-io.c b/drivers/ide/ide-io.c
> > index 7153796..bac5ea1 100644
> > --- a/drivers/ide/ide-io.c
> > +++ b/drivers/ide/ide-io.c
> > @@ -1595,6 +1595,7 @@ void ide_init_drive_cmd (struct request *rq)
> >  {
> >  	memset(rq, 0, sizeof(*rq));
> >  	rq->ref_count = 1;
> > +	rq->cmd = rq->__cmd;
> >  }
> >  
> >  EXPORT_SYMBOL(ide_init_drive_cmd);
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index b3a58ad..5710ae4 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -215,8 +215,9 @@ struct request {
> >  	/*
> >  	 * when request is used as a packet command carrier
> >  	 */
> > -	unsigned int cmd_len;
> > -	unsigned char cmd[BLK_MAX_CDB];
> > +	unsigned short cmd_len;
> > +	unsigned char __cmd[BLK_MAX_CDB];
> > +	unsigned char *cmd;
> >  
> >  	unsigned int data_len;
> >  	unsigned int extra_len;	/* length of alignment and padding */
> > @@ -812,6 +813,13 @@ static inline void put_dev_sector(Sector p)
> >  	page_cache_release(p.v);
> >  }
> >  
> > +static inline void rq_set_cmd(struct request *rq, unsigned char *cmd,
> > +			      unsigned short cmd_len)
> > +{
> > +	rq->cmd = cmd;
> > +	rq->cmd_len = cmd_len;
> > +}
> > +
> 
> This is 100% identical to what I suggested be done instead, so of course
> I agree with this.

Yeah, I think that it's the most straightforward. We have one value to
represent one item, the length of command. We can access to rq->cmd
and rq->cmd_len as before.

I don't see how it's is dangerous to have a single value to represent
the length of command though changing rq->cmd from the static array to
a pointer to support large commands is tricky. It's possible that I
overlooked something that uses the block layer in an uncommon way as
ide does.