Re: [PATCH] block: fix sg SG_DXFER_TO_FROM_DEV regression

[Douglas Gilbert <dgilbert@interlog.com>]

FUJITA Tomonori wrote:
> I overlooked SG_DXFER_TO_FROM_DEV support when I converted sg to use
> the block layer mapping API (2.6.28).
> 
> Douglas Gilbert explained SG_DXFER_TO_FROM_DEV:
> 
> http://www.spinics.net/lists/linux-scsi/msg37135.html
> 
> =
> The semantics of SG_DXFER_TO_FROM_DEV were:
>    - copy user space buffer to kernel (LLD) buffer
>    - do SCSI command which is assumed to be of the DATA_IN
>      (data from device) variety. This would overwrite
>      some or all of the kernel buffer
>    - copy kernel (LLD) buffer back to the user space.
> 
> The idea was to detect short reads by filling the original
> user space buffer with some marker bytes ("0xec" it would
> seem in this report). The "resid" value is a better way
> of detecting short reads but that was only added this century
> and requires co-operation from the LLD.
> =
> 
> This patch changes the block layer mapping API to support this
> semantics. This simply adds another field to struct rq_map_data and
> enables __bio_copy_iov() to copy data from user space even with READ
> requests.
> 
> It's better to add the flags field and kills null_mapped and the new
> from_user fields in struct rq_map_data but that approach makes it
> difficult to send this patch to stable trees because st and osst
> drivers use struct rq_map_data (they were converted to use the block
> layer in 2.6.29 and 2.6.30). Well, I should clean up the block layer
> mapping API.
> 
> zhou sf reported this regiression and tested this patch:
> 
> http://www.spinics.net/lists/linux-scsi/msg37128.html
> http://www.spinics.net/lists/linux-scsi/msg37168.html

Wrote my own little test program and it gave the wrong
result (as reported by zhou sf) in lk 2.6.30 . When
this patch was applied, the expected result (i.e. a
response with user supplied fill characters) was observed.

Here is an INQUIRY response set up for a length of
128 bytes with an 0xec fill character:

$ ./sg_dxfer_to_from
resid indicates that 96 bytes returned in response
  00     00 00 05 02 5b 00 10 0a  4c 69 6e 75 78 20 20 20
  10     73 63 73 69 5f 64 65 62  75 67 20 20 20 20 20 20
  20     30 30 30 34 00 00 00 00  00 00 00 00 00 00 00 00
  30     00 00 00 00 00 00 00 00  00 00 00 77 03 14 03 3d
  40     0c 0f 00 00 00 00 00 00  00 00 00 00 00 00 00 00
  50     00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
  60     ec ec ec ec ec ec ec ec  ec ec ec ec ec ec ec ec
  70     ec ec ec ec ec ec ec ec  ec ec ec ec ec ec ec ec


Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>