Re: [PATCH v2 5/5] scsi: ufs: UFS Host Performance Booster(HPB) driver

[Bart Van Assche <bvanassche@acm.org>]

On 2020-04-26 23:13, Avri Altman wrote:
>> On 2020-04-25 01:59, Avri Altman wrote:
> HPB support is comprised of 4 main duties:
> 1) Read the device HPB configuration
> 2) Attend the device's recommendations that are embedded in the sense buffer
> 3) L2P cache management - This entails sending 2 new scsi commands (opcodes were taken from the vendor pool):
> 	a. HPB-READ-BUFFER - read L2P physical addresses for a subregion
> 	b. HPB-WRITE-BUFFER - notify the device that a region is inactive (in host-managed mode)
> 4) Use HPB-READ: a 3rd new scsi command (again - uses the vendor pool) to perform read operation instead of READ10.  HPB-READ carries both the logical and the physical addresses.
> 
> I will let Bean defend the Samsung approach of using a single LLD to attend all 4 duties.
> 
> Another approach might be to split those duties between 2 modules:
>  - A LLD that will perform the first 2 - those can be done only using ufs privet stuff, and 
>  - another module in scsi mid-layer that will be responsible of L2P cache management,
>   and HPB-READ command setup.
> A framework to host the scsi mid-layer module can be the scsi device handler.
> 
> The scsi-device-handler infrastructure was added to the kernel mainly to facilitate multiple paths for storage devices.
> The HPB framework, although far from the original intention of the authors, might as well fit in.
> In that sense, using READs and HPB_READs intermittently, can be perceived as a multi-path.
> 
> Scsi device handlers are also attached to a specific scsi_device (lun).
> This can serve as the glue linking between the ufs LLD and the device handler which resides in the scsi level.
> 
> Device handlers comes with a rich and handy set of APIs & ops, which we can use to support HPB.
> Specifically we can use it to attach & activate the device handler,
> only after the ufs driver verified that HPB is supported by both the platform and the device. 
> 
> The 2 modules can communicate using the handler_data opaque pointer,
> and the handler’s set_params op-mode: which is an open protocol essentially,
> and we can use it to pass the sense buffer in its full or just a parsed version.
> 
> Being a scsi mid-layer module, it will not break anything while sending
> HPB-READ-BUFFER and HPB-WRITE-BUFFER as part of the L2P cache management duties.
> 
> Last but not least, the device handler is already hooked in the scsi command setup flow - scsi_setup_fs_cmnd(),
> So we get the hook into HPB-READ prep_fn for free.   
>  
> Later on, we might want to export the L2P cache management logic to user-space.
> Locating the L2P cache management in scsi mid-layer will enable us to do so, using the scsi-netlink or some other means.

Hi Avri,

I'm not sure that I agree that HPB can be perceived as multi-path.
Anyway, the above approach sounds interesting to me. A few questions though:
- The only in-tree caller of scsi_dh_attach() I am aware of exists in
  the dm-mpath driver. I think that call is triggered by multipathd.
  I don't think that it is acceptable to require that multipathd is
  running to use the UFS HPB functionality. What is the plan for
  attaching the UFS device handler to UFS devices?
- Will preparing a SCSI command involve executing a SCSI command? If so,
  how will it be prevented that execution of that internally submitted
  SCSI command triggers a deadlock due to tag exhaustion?

Thanks,

Bart.