[RFC PATCH 0/3] blk/scsi/ufs: scatter-gather read support

[Urvashi Sharma <urvashi.sharma@oss.qualcomm.com>]

This implements the scatter-gather read design discussed in [1].

Reading one contiguous block range into several separate buffers
currently takes one read request per buffer, since the block uclass
has no way to pass a list of destination addresses to a driver.
Controllers which accept such a list for a single command, like UFS
with its PRDT, cannot make use of it.

Patch 1 adds the block layer side: an optional read_sg() operation
taking a struct blk_sg_list, and blk_read_sg() to drive it.
blk_read_sg() checks that every entry is a non-zero multiple of the
block size and that the lengths add up to the requested block count,
then either calls the driver or loops blk_read() over the list, so
callers behave the same way regardless of the driver. Patches 2 and 3
implement read_sg() for SCSI and UFS.

Changes from the design posted in [1]:

- addr in struct blk_sg_entry is void * rather than unsigned int,
  and entries carry a byte length rather than a block count.

- The list is still a struct rather than a bare count argument, to
  leave room for flags or alignment hints without changing the
  read_sg() signature or its callers.

- Names are prefixed blk_sg_ / read_sg to keep the storage sense of
  scatter-gather distinct from the DMA one.

- blk_read_sg() falls back to one blk_read() per buffer when the
  driver has no read_sg(), or rejects a list with -ENOSYS or
  -EOPNOTSUPP, instead of returning -ENOSYS to the caller.

- Kconfig symbols are BLK_SCATTER_GATHER_READ and
  UFS_SCATTER_GATHER, as the generic part belongs to the block layer
  rather than SCSI.

There is no in-tree user yet, so both symbols default to off and no
defconfig selects them.

Tested on qemu_arm64 with QEMU's UFS device, reading a block range
into several buffers with blk_read_sg() via UFS_PCI.

[1] https://lore.kernel.org/u-boot/ptmm7jaltdwuyxj5ks26oht2cj6ckp4at6prvra27l4r6idkiz@xzyj2s3spx25/

Urvashi Sharma (3):
  blk: add scatter-gather read support
  scsi: add scatter-gather read support
  ufs: add scatter-gather read support

 drivers/block/Kconfig      | 10 +++++
 drivers/block/blk-uclass.c | 75 ++++++++++++++++++++++++++++++++++++++
 drivers/scsi/scsi.c        | 58 +++++++++++++++++++++++++++++
 drivers/ufs/Kconfig        |  8 ++++
 drivers/ufs/ufs-uclass.c   | 75 ++++++++++++++++++++++++++++++++++++--
 include/blk.h              | 61 +++++++++++++++++++++++++++++++
 include/scsi.h             |  6 +++
 7 files changed, 290 insertions(+), 3 deletions(-)

-- 
2.34.1