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

From: Douglas Gilbert
Date: Mon Feb 19 2007 - 17:25:36 EST

Alan Stern wrote:
> On Mon, 19 Feb 2007, Joerg Schilling wrote:
>> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>>> Well, if Doug wants to reduce the value returned by SG_GET_RESERVED_SIZE,
>>> it's okay with me. An advantage of doing this is that older versions of
>>> cdrecord would then work correctly.
>>> However you don't seem to realize that people can use programs like
>>> cdrecord with devices whose drivers don't support SG_GET_RESERVED_SIZE --
>>> because that ioctl works only with sg. Programs would have to try
>>> SG_GET_RESERVED_SIZE and if it faied, then try BLKSECTGET.
>> Is there any reason not to have one single ioctl for one basic feature?
> Indeed there is not. That's what I wrote in an earlier email:
> "There should be one single ioctl which can be applied uniformly to all
> CD-type devices (in fact, to all devices using a request_queue) to learn
> max_sectors. This rules out using SG_GET_RESERVED_SIZE."
>>> Remember also, the "reserved size" is _not_ the maximum allowed size of a
>>> DMA transfer. Rather, it is the size of an internal buffer maintained by
>>> sg. It's legal to do an I/O transfer larger than the "reserved size", but
>>> it is not legal to do an I/O transfer larger than max_sectors.
>> At the time the call SG_GET_RESERVED_SIZE has been discussed/defined, we did
>> originally agree that the max value should be limited to what the HW allows
>> as DMA size. This is why I did originally files a bug against
> How do you feel about the patch below, either in addition to or instead of
> the previous patch?

The SG_GET_RESERVED_SIZE ioctl is also defined in
the block layer, see block/scsi_ioctl.c .
I suspect it is just a kludge to fool cdrecord that it
is talking to a sg device. [One of many kludges in the
block SG_IO ioctl implementation to that end.]
So perhaps the block layer versions of SG_SET_RESERVED_SIZE
and SG_GET_RESERVED_SIZE need to be similarly capped.
Actually I think that I would default SG_GET_RESERVED_SIZE to
request_queue->max_sectors * 512 in the block layer
implementation (as there is no "reserve buffer" associated
with a block device).

The idea of a reserved buffer may live on in bsg as experience
with sg has shown that it is the fastest way to do (mmap-ed) IO.
Having one reserved buffer per file descriptor means not
having to create and tear down a scatter gather list
per IO. [Having a pool of such lists would be even better.]
Until optical storage needs 10 times its current datarates
then cdrecord will not need this mechanism.

Doug Gilbert

> Index: usb-2.6/drivers/scsi/sg.c
> ===================================================================
> --- usb-2.6.orig/drivers/scsi/sg.c
> +++ usb-2.6/drivers/scsi/sg.c
> @@ -917,6 +917,8 @@ sg_ioctl(struct inode *inode, struct fil
> return result;
> if (val < 0)
> return -EINVAL;
> + if (val > sdp->device->request_queue->max_sectors * 512)
> + return -EOVERFLOW;
> if (val != sfp->reserve.bufflen) {
> if (sg_res_in_use(sfp) || sfp->mmap_called)
> return -EBUSY;
> @@ -925,7 +927,8 @@ sg_ioctl(struct inode *inode, struct fil
> }
> return 0;
> - val = (int) sfp->reserve.bufflen;
> + val = min_t(int, sfp->reserve.bufflen,
> + sdp->device->request_queue->max_sectors * 512);
> return put_user(val, ip);
> result = get_user(val, ip);

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