[PATCH v4 0/3] scsi: ufs: Allow RTT negotiation

[Avri Altman]

The rtt-upiu packets precede any data-out upiu packets, thus
synchronizing the data input to the device: this mostly applies to write
operations, but there are other operations that requires rtt as well.

There are several rules binding this rtt - data-out dialog, specifically
There can be at most outstanding bMaxNumOfRTT such packets.  This might
have an effect on write performance (sequential write in particular), as
each data-out upiu must wait for its rtt sibling.

UFSHCI expects bMaxNumOfRTT to be min(bDeviceRTTCap, NORTT). However,
as of today, there does not appears to be no-one who sets it: not the
host controller nor the driver.  It wasn't an issue up to now:
bMaxNumOfRTT is set to 2 after manufacturing, and wasn't limiting the
write performance.

UFS4.0, and specifically gear 5 changes this, and requires the device to
be more attentive.  This doesn't come free - the device has to allocate
more resources to that end, but the sequential write performance
improvement is significant. Early measurements shows 25% gain when
moving from rtt 2 to 9. Therefore, set bMaxNumOfRTT to be
min(bDeviceRTTCap, NORTT) as UFSHCI expects.

v3 -> v4:
Allow bMaxNumOfRTT to be configured via sysfs (Bart)

v2 -> v3:
Allow platform vendors to take precedence having their own rtt
negotiation mechanism (Peter)

v1 -> v2:
bMaxNumOfRTT is a Persistent attribute - do not override if it was
written (Bean)

Avri Altman (3):
  scsi: ufs: Allow RTT negotiation
  scsi: ufs: Allow platform vendors to set rtt
  scsi: ufs: sysfs: Make max_number_of_rtt read-write

 Documentation/ABI/testing/sysfs-driver-ufs | 14 +++---
 drivers/ufs/core/ufs-sysfs.c               | 58 +++++++++++++++++++++-
 drivers/ufs/core/ufshcd.c                  | 39 +++++++++++++++
 include/ufs/ufshcd.h                       |  4 ++
 include/ufs/ufshci.h                       |  1 +
 5 files changed, 109 insertions(+), 7 deletions(-)

-- 
2.34.1