James Bottomley wrote: > On Wed, 2006-12-06 at 11:32 -0500, Alan Stern wrote: >>> But how did he get the file descriptor? He opened a device name, which >>> could have been used to get the sysfs file. >> The device name was probably something like /dev/sg0. This doesn't easily >> permit one to find the corresponding sysfs filename for the real >> underlying device, although it can be done with difficulty. (That's why I >> used the excessively-ornate sysfs pathname in the Bugzilla entry.) It >> certainly wouldn't be as easy as using an ioctl. >> >> It wouldn't be as uniform either. The search through sysfs would have to >> be different depending on whether the device name was /dev/sr0 or >> /dev/sg0. > > Realistically, no-one makes SCSI CDs or DVDs any more ... I know, I've > tried to get some for some of my older boxes. Most of them nowadays are > IDE attachments, which don't have a /dev/sg node. So /dev/sg is really > the legacy mode for burning. The correct way to do it today is to use > the actual device name ... then you don't have to worry about what the > transport is any more. All CD and DVD drive these days use SCSI. That is SCSI command sets: MMC and SPC. Very few use the SCSI Parallel Interface (SPI). An increasing number will be using S-ATAPI and they could be seen by the OS via "SCSI" transports: FC and SAS (+ SATA). >>>> Is the patch below acceptable? >>> Really, no. The parameter you're fishing for is a block parameter, not >>> a SCSI parameter ... it should really be a block ioctl if we have to >>> have an ioctl at all. >> I could easily enough rewrite the patch to put the ioctl somewhere else >> (although I'm not quite sure exactly where would be best). But do >> non-block devices have request queues? What about the points that Doug >> raised: > > All CD/DVD burners are block devices, which is the problem set under > discussion. CD/DVD burners are block device for read operations only. When they are "burning" they are not block devices in the normal sense. So if this was classic Unix a block device node would be used for reading and a raw device node for writing. Just like .... I'm wasting keystrokes. >> On Tue, 5 Dec 2006, Douglas Gilbert wrote: >> >>> Apart from sensibly yielding the max size in bytes, your patch >>> has the added benefit of allowing non-block devices (e.g. tape, >>> processor and enclosure services) to find out what limit the >>> OS/host has placed on each command's maximum transfer size. > > They all possess block queues, yes, so we should really allow access to > the block ioctls for them. > >>> If you manage to get that ioctl in, then ungrateful people >>> will ask for the corresponding "set" operation as well. >>> >>> >>> To illustrate the /sys mess look at naming of the sysfs approach >>> to this problem. For example: >>> /sys/block/sde/queue/max_sectors_kb >>> - it is not only a "block" property >>> - sde is an "end device" and suggests information from that >>> device's Block Limits VPD page, actually it is a limit >>> imposed by the OS and the host used to access that device >>> - what has "queue" got to do with it? >>> - "max_sectors_kb" should have units of bytes >> In addition to all of these points, there remains the peculiar location of >> the SG_ ioctls. They are implemented it two places in the kernel: >> block/scsi_ioctl.c and drivers/scsi/sg.c. And the two implementations of >> e.g. SG_SET_RESERVED_SIZE don't even do the same thing! > > I have no idea why the block layer even implements > SG_SET_RESERVED_SIZE ... I suspect it was some legacy application > compatibility problem, so it probably can be eliminated. It was put there to trick cdrecord! Doug Gilbert - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
