Boaz Harrosh wrote:
Vladislav Bolkhovitin wrote:
FUJITA Tomonori wrote:
This patchset removes the majority of scsi_execute_async in st
driver. IOW, this converts st driver to use the block layer functions.
We are in the process of removing scsi_execute_async and
scsi_req_map_sg. scsi_execute_async in sg driver were removed in
2.6.27. Now only st and osst drivers use scsi_execute_async().
st driver uses scsi_execute_async for two purposes, performing sort
management SCSI commands internally and data transfer between user and
kernel space (st_read and st_write). This patchset converts the
former.
The former performs SCSI commands synchronously. It uses a liner
in-kernel buffer (not scatter gather) for data transfer. To perform
such commands, other upper layer drivers (e.g. sd) use
scsi_execute_req (internally uses blk_rq_map_kern and and
blk_execute_rq). scsi_execute_req is not fully fit for st so I adds a
helper function similar to scsi_execute_req and replace
scsi_execute_async in st driver (I might modify scsi_execute_req for
st and remove the helper function later).
I'm still working on converting scsi_execute_async for data transfer
between user and kernel space but I want to get this first half be
merged.
May I ask a possibly stupid question? Is amount of overall code in Linux
SCSI subsystem after your conversion is fully done and
scsi_execute_async() with scsi_req_map_sg() completely removed going to
get smaller or bigger?
From diffstats of your patches seems it's is going to get considerably
bigger (171 insertion vs 61 deletion). And this is only in one user of
scsi_execute_async() of 4. What's the point then in scsi_execute_async()
removal if the resulting code is going to get bigger, not smaller? And,
frankly, it doesn't look as it's going to get clearer too..
P.S. Scsi_execute_async() is just ~50 lines long, scsi_req_map_sg() is
just ~80 lines long and both look like nice helper functions, which are
not worse than st_scsi_kern_execute().
Regards,
Vlad
No! you are not looking at the full picture. See previous conversion of
sg.c driver. What you see here is only the rq_map_kern side which is
fine. But the ugliness starts with when you need sg-lists.
With Scsi_execute_async you go from:
memory=>sg-lists=>bios=>sg-lists=>driver
going directly to blk you save the allocation of the first sg-list and
translation to BIOs. And the ugly self-made preparation of the sg-list
by each driver again and again. Not talking about correctness. Like when
the block layer takes care of alignment, draining, sizes and all that funny
stuff all kind of drivers need (sa*cough*ta)
The net effect is a very big gain. both in code and specially in resources
and performance.
Note that all users of block layer have user-memory, kernel-pointer, page*.
Only target driver happens to be on the other side and actually hold an sg-list
at hand. That's because, in principle, it is down-side-up.
I see. If it's about performance, it should be done, no doubts. But I
have another likely stupid question, answer for which I have been going
to find out for long time, but always have no time. Maybe, you can help
me, BTW, please? Why is block layer involved in SCSI commands execution
for non-block cases, like st and sg? Sg isn't intended to be actively
used for data transfers with block devices, for them there are sd and
bsg interfaces, right? Wouldn't for them path memory=>sg-lists=>driver
via SCSI mid-layer be simpler and all those alignment, draining, sizes,
etc. issues can't be handled by some library, which both SCSI and block
subsystems use? I have feeling, maybe wrong, that currently performance
for non-block cases is sacrificed for block case.
Thanks,
Vlad
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