Re: [PATCH RESEND 2/4] scsi: sg: implement BLKSSZGET

If we rename it to e.g. SG_GET_MAX_XFER_BYTES, it will still break
applications unless we also keep the wrong/ugly/confusing name (and
you lose the advantage/generality that the two ioctls can be used on
both sg and "pure" block devices; which seems to be the case of some
SG_* ioctls as well).

I don't see what it has to do with passthrough. Either way, it's just
a matter of whether you want to decouple it and make things more
flexible. The only real disadvantage is, you will have to do two
ioctls instead of one, but no more than that, and for good reasons.

I don't really care enough though. I mean, I'm okay with
SG_GET_MAX_XFER_BYTES *and* NO "improper" BLKSECTGET. If that will get
the patch series in, I am willing to send a new version. If not, I'm
just gonna drop this.

On Tue, 8 Sep 2020 at 16:43, Christoph Hellwig [off-list ref] wrote:

On Mon, Sep 07, 2020 at 05:01:34PM +0800, Tom Yan wrote:

quoted

Feel free to omit this. But then you will probably want to ditch
BLKSECTGET as well, and then any usage of queue_max_sectors(), and
maybe more/all queue_*().

I'm not really interested in discussing/arguing whether
general/ideally-speaking it's appropriate/necessary to keep BLKSECTGET
/ add BLKSSZGET. The only reason I added this is that, when BLKSECTGET
was introduced to sg long time ago, it was wrongly implemented to
gives out the limit in bytes, so now when I'm fixing it, I'm merely
making sure that whatever has been relying on the ioctl (e.g. qemu)
will only need to do one more ioctl (instead of e.g. doing SCSI in its
non-SCSI-specific part), if they want/need the limit in bytes. If they
can be implemented more "generic"-ly, feel free to improve/extend them
to make them "SG_*-qualified".

Even if you can do SCSI from the userspace, or even should, I don't
see any reason that we shouldn't provide an ioctl to do
queue_logical_block_size() *while we provide one to do
queue_max_sectors()*.

Well, the different definition in bytes for sg actually makes sense
to me, as a bytes based limit is what fundamentally makes sense for
the passthrough interface.  Only that it reuses the same cmd value
is a bit confusing.  So instead of changing anything and potentially
breaking applications I'd suggest to just better document the semantics.