Re: [PATCH 2/2] block: Split and submit bios in LBA order

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 3/23/23 17:26, Christoph Hellwig wrote:
> On Tue, Mar 21, 2023 at 07:36:12AM -0700, Bart Van Assche wrote:
>> The UFSHCI specification is very clear about the requirement that UFS host 
>> controllers must process SCSI commands in order if host software sets one 
>> bit at a time in the UFSHCI 3.0 doorbell register: "For Task Management 
>> Requests and Transfer Requests, software may issue multiple commands at a 
>> time, and may issue new commands before previous commands have completed. 
>> When software sets the corresponding doorbell register, the Task Management 
>> Requests and Transfer Requests automatically get a time stamp with their 
>> issue time. The commands within a command list (Task Management List or 
>> Transfer Request List) shall be processed in
>> the order of their time stamps, starting from the oldest time stamp. In the 
>> case multiple commands from the same list have the same time stamp, they 
>> shall be processed in the order of their command list index,
>> starting from the lowest index."
> 
> But we can't write Linux software just for UFS.  We have no sensible
> ordering guarantee anywhere else.
> 
>> Damien and Jens agree about introducing an additional hardware queue for 
>> preserving the order of zoned writes as one can see here: 
>> https://lore.kernel.org/linux-block/ed255a4a-a0da-a962-2da4-13321d0a75c5@kernel.dk/
>>
>> In our tests pipelining zoned writes (REQ_OP_WRITE) works fine as long as 
>> the UFS error handler is not activated. After the UFS error handler has 
>> been scheduled and before the SCSI host state is changed into 
>> SHOST_RECOVERY, the UFS host controller driver responds with 
>> SCSI_MLQUEUE_HOST_BUSY. I'm still working on a solution for the reordering 
>> caused by this mechanism.
> 
> We'll still need REQ_OP_ZONE_APPEND as the actual file system fast path
> interface.  For a low-end device like UFS the sd.c emulation might be
> able to take advantage of the above separate queue as an implementation
> detail.

For the zone append emulation, the write locking is done by sd.c and the upper
layer does not restrict to one append per zone. So we actually could envision a
UFS version of the sd write locking calls that is optimized for the device
capabilities and we can keep a common upper layer (which is preferable in my
opinion).

-- 
Damien Le Moal
Western Digital Research