Questions about probing WLUNs for UFS power management
Date: Wed, 24 Sep 2025 02:32:51 UTC
Hello freebsd-scsi,
I’m trying to implement Well-known LUNs (WLUNs) for the UFS (Universal
Flash Storage, ufshci(4)) driver, but the current SCSI XPT doesn't probe
WLUNs, so I have a few questions.
First, Well-known LUNs are defined in SCSI SAM-5. In UFS, 'REPORT LUNS',
'UFS Device', 'RPMB', and 'BOOT' LUNs are treated as WLUNs.
(SAM-5 4.7.5.1 "Well-known logical unit addressing",
UFS 2.1 spec 10.8.5 "10.8.5 Well Known Logical Unit Defined in UFS")
Because a UFS device contains multiple LUNs, power management requires
sending a START STOP UNIT(SSU) command to the 'UFS Device' WLUN. (If SSU
is sent to a non-WLUN (normal) LUN, the device power mode is not changed
and only that LUN becomes disabled.)
(UFS 2.1 spec 7.4.2 "Power Management Command: START STOP UNIT")
In FreeBSD, the SCSI XPT logic that parses LUNs checks the 'Simple logical
unit addressing format' (SAM-5 Table 26) and the 8-byte 'Extended logical
unit addressing format' (SAM-5 Table 35), but it does not check the
'Well-known logical unit extended addressing format' (SAM-5 Table 37).
In the scsi_xpt.c code below, CAM_GET_LUN reads Extended LUNs and
CAM_GET_SIMPLE_LUN reads Simple LUNs:
```
sys/cam/scsi/scsi_xpt.c line:2114
static void
scsi_scan_bus(struct cam_periph *periph, union ccb *request_ccb)
...
while (scan_info->lunindex[target_id] < nluns) {
if (scan_info->cpi->hba_misc & PIM_EXTLUNS) {
CAM_GET_LUN(target->luns,
scan_info->lunindex[target_id],
lun_id);
break;
}
if (CAM_CAN_GET_SIMPLE_LUN(target->luns,
scan_info->lunindex[target_id])) {
CAM_GET_SIMPLE_LUN(target->luns,
scan_info->lunindex[target_id],
lun_id);
break;
}
scan_info->lunindex[target_id]++;
}
```
Also, we can obtain the number of WLUNs via the REPORT LUNS SCSI command.
To retrieve WLUNs, the SELECT REPORT field must be set to 0x01, but
SCSI XPT currently uses 0x00 to obtain only normal LUNs.
(UFS 2.1 spec 11.3.12.2 "Report LUNS Command Select Report Field Values")
```
sys/cam/scsi/scsi_xpt.c line:827
static void
probestart(struct cam_periph *periph, union ccb *start_ccb)
...
case PROBE_REPORT_LUNS:
...
scsi_report_luns(csio, 5, probedone, MSG_SIMPLE_Q_TAG,
RPL_REPORT_DEFAULT, rp, periph->path->target->rpl_size,
SSD_FULL_SIZE, 60000);
```
```
sys/cam/scsi/scsi_all.h line:3020
struct scsi_report_luns
{
uint8_t opcode;
uint8_t reserved1;
#define RPL_REPORT_DEFAULT 0x00
#define RPL_REPORT_WELLKNOWN 0x01
```
Looking at the Linux code, Linux also does not check WLUNs at the SCSI
layer. Therefore, the Linux UFS driver explicitly registers the WLUNs
during initialization by calling __scsi_add_device()
```
drivers/ufs/core/ufshcd.c line:8137
/**
* ufshcd_scsi_add_wlus - Adds required W-LUs
* @hba: per-adapter instance
*
* UFS device specification requires the UFS devices to support 4 well known
* logical units:
* "REPORT_LUNS" (address: 01h)
* "UFS Device" (address: 50h)
* "RPMB" (address: 44h)
* "BOOT" (address: 30h)
* UFS device's power management needs to be controlled by "POWER CONDITION"
* field of SSU (START STOP UNIT) command. But this "power condition" field
* will take effect only when its sent to "UFS device" well known logical unit
* hence we require the scsi_device instance to represent this logical unit in
* order for the UFS host driver to send the SSU command for power management.
*
* We also require the scsi_device instance for "RPMB" (Replay Protected Memory
* Block) LU so user space process can control this LU. User space may also
* want to have access to BOOT LU.
*
* This function adds scsi device instances for each of all well known LUs
* (except "REPORT LUNS" LU).
*
* Return: zero on success (all required W-LUs are added successfully),
* non-zero error value on failure (if failed to add any of the required W-LU).
*/
static int ufshcd_scsi_add_wlus(struct ufs_hba *hba)
{
int ret = 0;
struct scsi_device *sdev_boot, *sdev_rpmb;
hba->ufs_device_wlun = __scsi_add_device(hba->host, 0, 0,
ufshcd_upiu_wlun_to_scsi_wlun(UFS_UPIU_UFS_DEVICE_WLUN), NULL);
if (IS_ERR(hba->ufs_device_wlun)) {
ret = PTR_ERR(hba->ufs_device_wlun);
hba->ufs_device_wlun = NULL;
goto out;
}
scsi_device_put(hba->ufs_device_wlun);
sdev_rpmb = __scsi_add_device(hba->host, 0, 0,
ufshcd_upiu_wlun_to_scsi_wlun(UFS_UPIU_RPMB_WLUN), NULL);
if (IS_ERR(sdev_rpmb)) {
ret = PTR_ERR(sdev_rpmb);
goto remove_ufs_device_wlun;
}
ufshcd_blk_pm_runtime_init(sdev_rpmb);
scsi_device_put(sdev_rpmb);
sdev_boot = __scsi_add_device(hba->host, 0, 0,
ufshcd_upiu_wlun_to_scsi_wlun(UFS_UPIU_BOOT_WLUN), NULL);
if (IS_ERR(sdev_boot)) {
dev_err(hba->dev, "%s: BOOT WLUN not found\n", __func__);
} else {
ufshcd_blk_pm_runtime_init(sdev_boot);
scsi_device_put(sdev_boot);
}
goto out;
remove_ufs_device_wlun:
scsi_remove_device(hba->ufs_device_wlun);
out:
return ret;
}
```
---------------------------------------------------------------------------
My question is: which approach would be more appropriate for using WLUN
devices in UFS driver on FreeBSD?
(1) Add support in SCSI XPT to probe WLUNs and allocate devices for them.
(2) Follow Linux: have the UFS driver explicitly probe/register WLUN
devices itself.
If (2) is preferred, what is the recommended way for the UFSHCI driver to
probe/register WLUNs in FreeBSD? (Is there an equivalent to Linux’s
__scsi_add_device()?)
Thanks,
Jaeyoon