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

From: Alan Stern
Date: Mon Feb 19 2007 - 12:06:52 EST

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?

Alan Stern

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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