linux-scsi-owner@xxxxxxxxxxxxxxx wrote on 03/06/2005 03:03:04: > James Bottomley wrote: > > On Thu, 2005-06-02 at 20:31 +0300, Liran Schour wrote: > > > >>I am developing a new SCSI upper layer driver. (Object based SCSI device) > >>I noticed that scsi_do_req execute commands in LIFO order (add new commands > >>to the head of the Q). > >>This behavior can cause a starvation of commands (or at least long delays). > >>What is the proper solution to this? > > > > > > Well, to use the proper interfaces. > > > > scsi_do_req is deprecated, so don't use it. scsi_wait_req is its > > replacement. > > James, > scsi_wait_req() doesn't look like a replacement of > scsi_do_req() to me. The "wait" call is synchronous > while scsi_do_req() is asynchronous with a done() > callback. > > Note: they're only really intended to allow ULDs to > > execute commands that are necessary for controlling the device (that's > > why they go at the head of the queue). > > The "char" ULDs use scsi_do_req() exclusively for > SCSI command injection. The sg driver could be sharing > the device with sd driver, but then again it could > be talking to a SES target. The block layer SG_IO ioctl > is the only use in "mixed" mode (e.g. smartmontools > usage). However the block layer SG_IO is not being > used in "mixed" mode when a CD/DVD is being burnt. > IMO the mid level should not arbitrarily adopt LIFO > order. > > Block commands are executed in > > order using ULD init_command() (or REQ_BLOCK_PC). > > OSDs break the UNIX block paradigm. Perhaps > it's time to start thinking about how to properly > support them. > > Doug Gilbert In order to run our OSD SCSI initiator on top of the Linux SCSI layer we have needed to do the following changes to the SCSI code: 1. Redefine MAX_COMMAND_SIZE to 256 byte. 2. Define a new scatter gather structures that includes two virtual buffers on for data out and one for data in. (implementing bidirectional commands) The SCSI middle layer doesn't now this new structure (only ULD and LLD). 3. The problem above - define a new scsi_do_req that inserts our commands to the tail of the Q. We intend to release our code as an open source project soon. - Liran - : 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
