Re: [PATCH] Block layer: separate out queue-oriented ioctls

[Joerg Schilling]

Douglas Gilbert <dougg@xxxxxxxxxx> wrote:

> Jens Axboe wrote:
> > On Fri, Feb 16 2007, Alan Stern wrote:
> >> From: James Bottomley <James.Bottomley@xxxxxxxxxxxx>
> >>
> >> This patch (as854) separates out the two queue-oriented ioctls from
> >> the rest of the block-layer ioctls.  The idea is that they should
> >> apply to any driver using a request_queue, even if the driver doesn't
> >> implement a block-device interface.  The prototypical example is the
> >> sg driver, to which the patch adds the new interface.
> >>
> >> This will make it possible for cdrecord and related programs to
> >> retrieve reliably the max_sectors value, regardless of whether the
> >> user points it to an sr or an sg device.  In particular, this will
> >> resolve Bugzilla entry #7026.
> > 
> > The block bits are fine with me, the sg calling point is a bit of a sore
> > thumb (a char driver calling into block layer ioctls) though. So the
> > block layer bits are certainly ok with me, if Doug acks the sg bit I'll
> > merge everything up.
> > 
> > (patch left below)
>
> Does this need to be in the sg driver?
>
> What is the hardware sector size of a SES or OSD device?
>
> As for the max_sector variable, wouldn't it be better
> to generate a new ioctl that yielded the limit in bytes?
> Making a driver variable that implicitly assumes sectors
> are 512 bytes in length more visible to the user space
> seems like a step in the wrong direction.

This is what I did propose. I know of no SCSI device made since 1986 that
has a "hardware sector size". This is really a DMA size limit in bytes
and if you return the number in an unrelated multiple of a fraction, you
will not be able to use the optmium max transfer size.

> Alternatively the SG_GET_RESERVED_SIZE ioctl could be
> modified to yield no more than max_sectors*512 .

This is what I did propose 3 months ago and already 2 years ago.

Jörg

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