Universal Flash Storage

1. Overview

Universal Flash Storage (UFS) is a storage specification for flash devices.It aims to provide a universal storage interface for bothembedded and removable flash memory-based storage in mobiledevices such as smart phones and tablet computers. The specificationis defined by JEDEC Solid State Technology Association. UFS is basedon the MIPI M-PHY physical layer standard. UFS uses MIPI M-PHY as thephysical layer and MIPI Unipro as the link layer.

The main goals of UFS are to provide:

  • Optimized performance:

    For UFS version 1.0 and 1.1 the target performance is as follows:

    • Support for Gear1 is mandatory (rate A: 1248Mbps, rate B: 1457.6Mbps)

    • Support for Gear2 is optional (rate A: 2496Mbps, rate B: 2915.2Mbps)

    Future version of the standard,

    • Gear3 (rate A: 4992Mbps, rate B: 5830.4Mbps)

  • Low power consumption

  • High random IOPs and low latency

2. UFS Architecture Overview

UFS has a layered communication architecture which is based on SCSISAM-5 architectural model.

UFS communication architecture consists of the following layers.

2.1 Application Layer

The Application layer is composed of the UFS command set layer (UCS),Task Manager and Device manager. The UFS interface is designed to beprotocol agnostic, however SCSI has been selected as a baselineprotocol for versions 1.0 and 1.1 of the UFS protocol layer.

UFS supports a subset of SCSI commands defined by SPC-4 and SBC-3.

  • UCS:

    It handles SCSI commands supported by UFS specification.

  • Task manager:

    It handles task management functions defined by theUFS which are meant for command queue control.

  • Device manager:

    It handles device level operations and deviceconfiguration operations. Device level operations mainly involvedevice power management operations and commands to Interconnectlayers. Device level configurations involve handling of queryrequests which are used to modify and retrieve configurationinformation of the device.

2.2 UFS Transport Protocol (UTP) layer

The UTP layer provides services forthe higher layers through Service Access Points. UTP defines 3service access points for higher layers.

  • UDM_SAP: Device manager service access point is exposed to devicemanager for device level operations. These device level operationsare done through query requests.

  • UTP_CMD_SAP: Command service access point is exposed to UFS commandset layer (UCS) to transport commands.

  • UTP_TM_SAP: Task management service access point is exposed to taskmanager to transport task management functions.

UTP transports messages through UFS protocol information unit (UPIU).

2.3 UFS Interconnect (UIC) Layer

UIC is the lowest layer of the UFS layered architecture. It handlesthe connection between UFS host and UFS device. UIC consists ofMIPI UniPro and MIPI M-PHY. UIC provides 2 service access pointsto upper layer:

  • UIC_SAP: To transport UPIU between UFS host and UFS device.

  • UIO_SAP: To issue commands to Unipro layers.

3. UFSHCD Overview

The UFS host controller driver is based on the Linux SCSI Framework.UFSHCD is a low-level device driver which acts as an interface betweenthe SCSI Midlayer and PCIe-based UFS host controllers.

The current UFSHCD implementation supports the following functionality:

3.1 UFS controller initialization

The initialization module brings the UFS host controller to active stateand prepares the controller to transfer commands/responses betweenUFSHCD and UFS device.

3.2 UTP Transfer requests

Transfer request handling module of UFSHCD receives SCSI commandsfrom the SCSI Midlayer, forms UPIUs and issues the UPIUs to the UFS Hostcontroller. Also, the module decodes responses received from the UFShost controller in the form of UPIUs and intimates the SCSI Midlayerof the status of the command.

3.3 UFS error handling

Error handling module handles Host controller fatal errors,Device fatal errors and UIC interconnect layer-related errors.

3.4 SCSI Error handling

This is done through UFSHCD SCSI error handling routines registeredwith the SCSI Midlayer. Examples of some of the error handling commandsissues by the SCSI Midlayer are Abort task, LUN reset and host reset.UFSHCD Routines to perform these tasks are registered withSCSI Midlayer through .eh_abort_handler, .eh_device_reset_handler and.eh_host_reset_handler.

In this version of UFSHCD, Query requests and power managementfunctionality are not implemented.

4. BSG Support

This transport driver supports exchanging UFS protocol information units(UPIUs) with a UFS device. Typically, user space will allocatestructufs_bsg_request andstructufs_bsg_reply (see ufs_bsg.h) asrequest_upiu and reply_upiu respectively. Filling those UPIUs shouldbe done in accordance with JEDEC spec UFS2.1 paragraph 10.7.Caveat emptor: The driver makes no further input validations and sends theUPIU to the device as it is. Open the bsg device in /dev/ufs-bsg andsend SG_IO with the applicable sg_io_v4:

io_hdr_v4.guard = 'Q';io_hdr_v4.protocol = BSG_PROTOCOL_SCSI;io_hdr_v4.subprotocol = BSG_SUB_PROTOCOL_SCSI_TRANSPORT;io_hdr_v4.response = (__u64)reply_upiu;io_hdr_v4.max_response_len = reply_len;io_hdr_v4.request_len = request_len;io_hdr_v4.request = (__u64)request_upiu;if (dir == SG_DXFER_TO_DEV) {        io_hdr_v4.dout_xfer_len = (uint32_t)byte_cnt;        io_hdr_v4.dout_xferp = (uintptr_t)(__u64)buff;} else {        io_hdr_v4.din_xfer_len = (uint32_t)byte_cnt;        io_hdr_v4.din_xferp = (uintptr_t)(__u64)buff;}

If you wish to read or write a descriptor, use the appropriate xferp ofsg_io_v4.

The userspace tool that interacts with the ufs-bsg endpoint and uses itsUPIU-based protocol is available at:

For more detailed information about the tool and its supportedfeatures, please see the tool’s README.

UFS specifications can be found at:

5. UFS Reference Clock Frequency configuration

Devicetree can define a clock named “ref_clk” under the UFS controller nodeto specify the intended reference clock frequency for the UFS storageparts. ACPI-based system can specify the frequency using ACPIDevice-Specific Data property named “ref-clk-freq”. In both ways the valueis interpreted as frequency in Hz and must match one of the values given inthe UFS specification. UFS subsystem will attempt to read the value whenexecuting common controller initialization. If the value is available, UFSsubsystem will ensure the bRefClkFreq attribute of the UFS storage device isset accordingly and will modify it if there is a mismatch.