Re: Questions about probing WLUNs for UFS power management
Date: Sun, 28 Sep 2025 21:48:06 UTC
Please excuse the tardy response
On Tue, Sep 23, 2025 at 8:32 PM Jaeyoon Choi <jaeyoon@freebsd.org> wrote:
> 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()?)
>
[[ I didn't trim context here, since it lays out the problem nicely). ]]
So I can make a case for either (1) or (2).
It's easy enough to add the enumeration for (1) to scsi_xpt.c It's just a
few more states in the probe state machine (which is a bit complex, but not
terribly so). If we treat them as separate LUNs, then a lot of the
infrastructure would just work. The key, though, is how do we keep scsi_da,
etc from attaching to these new nodes. And would we have generic scsi_foo.c
drivers for them (so the devices would get their own periph drivers) or
whether there's a higher-level device that would know it could use these
devices if it finds them and it wouldn't have a specific specialized kernel
drive for them, but instead use a generalized pass thru device.
I guess a lot of that would depend where things like power management is
happening. If it's in the UFS sim, that would suggest a different design
than if this was some super generic thing that would respond to generic
events that maybe the SIMs generate or maybe somebody else.
If we wanted to go with (1) heavily, we'd need new, generic periph drivers
along with some kind of way to get them to respond to, or report out events
in the system or with the drive. With only a couple of examples, this might
be difficult to abstract properly.
If we wanted to go with (2), then we'd likely need some extensions in xpt
around this so that the sim could discover, manage and use the WLUN devices.
Though the more I think about it, maybe it doesn't matter: we want to do
the WLUN probing regardless. It's who manages the cam_ed structures after
that. Where does the periph driver live, etc.
I think I need to go read the UFS specs in more detail than I have time for
this afternoon...
Warner
Warner