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

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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@xxxxxxxxx/
>
> 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.



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