Douglas Gilbert wrote: > Adar Dembo wrote: > > When sending SG_IO ioctls to a device: > > - /dev/sgX devices (using sg.c) follow the behavior described at > > http://tldp.org/HOWTO/SCSI-Generic-HOWTO/dio.html in terms of > > defaulting to indirect I/O, and using direct I/O only if both > > /proc/scsi/sg/allow_dio and SG_FLAG_DIRECT_IO in the request are set. > > - /dev/srX devices (using sr.c) default to direct I/O. At least, that > > appears to be the case from inspecting the code, as > > sr_block_ioctl->cdrom_ioctl->scsi_cmd_ioctl->sg_io->blk_rq_map_user- > > >__blk_rq_map_user will attempt to map the pages, regardless of what > > the user specified in the request or in the proc node. > > > > This difference caught me by surprise; my expectation would be that an > > sg_io_hdr_t passed via SG_IO would be serviced in the same way > > regardless of the underlying driver. Is this a bug, or a feature? If > > the latter, is it documented somewhere? > > As the name suggests the SG_IO ioctl was originally > implemented in the sg driver. Due to difficulties in > discovering which sg device corresponded to a particular > SCSI device node belonging to another driver (e.g. /dev/sr0 > belonging to the sr driver), the SG_IO ioctl was > grafted on to various other drivers (in some cases via > the block layer). > > The result is not always pretty and there are many > subtle differences. There is no 100% clean SCSI > pass-through in Linux (and if there was it could not > easily share a device with another high level SCSI driver > such as sr). The sg and the bsg drivers give the cleanest > pass-throughs. The rest tend to make assumptions appropriate > to their class of devices. And it is not just the SG_IO > ioctl that you need to take care of: some other drivers > will hang on open() if the device is not ready and the > O_NONBLOCK flag is not given. > > Summary: the driver "owning" the file descriptor that > the SG_IO ioctl uses, is in control. Thanks for the response. What you've described seems like a reasonable justification for why the different drivers should be allowed to handle SG_IO differently. That said, do you think it makes sense for the block layer's implementation to be extended to check allow_dio and/or SG_FLAG_DIRECT_IO, as the sg driver does? -- 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
