Re: [PATCH] scsi: ufs: mark HPB support as BROKEN

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 27, 2021 at 11:44:04AM -0400, Martin K. Petersen wrote:
> 
> Ming,
> 
> > request with scsi_cmnd may be allocated by the ufshpb driver, even it
> > should be fine to call ufshcd_queuecommand() directly for this driver
> > private IO, if the tag can be reused. One example is scsi_ioctl_reset().
> 
> scsi_ioctl_reset() allocates a new request, though, so that doesn't
> solve the forward progress guarantee. Whereas eh puts the saved request
> on the stack.

What I meant is to use one totally ufshpb private command allocated from
private slab to replace the spawned request, which is sent to ufshcd_queuecommand()
directly, so forward progress is guaranteed if the blk-mq request's tag can be
reused for issuing this private command. This approach takes a bit effort,
but avoids tags reservation.

Yeah, it is cleaner to use reserved tag for the spawned request, but we
need to know:

1) how many queue depth for the hba? If it is small, even 1 reservation
can affect performance.

2) how many inflight write buffer commands are to be supported? Or how many
is enough for obtaining expected performance? If the number is big, reserved
tags can't work.



Thanks,
Ming