On Sat, 1 Feb 2014, Steph Nguyen wrote: > > On Sat, 1 Feb 2014, Steph Nguyen wrote: > > > >> Hi > >> > >> It seems that when sending SCSI commands over USB, the kernel doesn't > >> necessary build a sense response. For instance, when sending REQUEST > >> SENSE over ehci, the scsi_cmnd->sense_buffer is left NULL; if I run the > >> same REQUEST SENSE command over a direct SCSI connect, the kernel builds > >> the scsi_cmnd->sense_buffer. > > > > There's no need for the kernel to build a sense buffer when sending a > > REQUEST SENSE command. That command always succeeds, so there's no > > reason to ask for sense data which might explain why the command > > failed. > > I'm using sg_requests. When running it against a disk attached to a usb > port, the sense_buffer is simply empty. When run against the same disk > directly attached to a scsi controller, the sense_buffer is built by the > kernel. So same device, same command, two different hw paths lead to two > different behaviors. Hold on a second. Above you said "build a sense response" and "scsi_cmnd->sense_buffer is left NULL". Those are two different things. In one case we would have scsi_cmnd->sense_buffer == NULL. In the other case, we would have scsi_cmnd->sense_buffer != NULL and scsi_cmnd->sense_buffer[i] = 0 for each i. Which do you really mean? > > In general, usb-storage doesn't touch scsi_cmnd except to set > > scsi_cmnd->result. (There are a few exceptions, but this is mostly > > true.) > Right, it makes sense that the scsi subsystem takes care of this. But, > looking at drivers/usb/storage/cypress_atacb.c, I can see that the sense > buffer for ATA_16 is built by the kernel. So I suppose this is one of > these exceptions you are referring to. No. Even in cypress_atacb.c, usb-storage does not assign anything to scsi_cmnd->sense_buffer. That is, it does not do anything like scsi_cmnd->sense_buffer = X; However, it does fill in the contents of the sense buffer. That's as it should be; usb-storage fills in the sense buffer contents for any command that completes abnormally. The reason you didn't see anything get filled in for your REQUEST SENSE command is because the command completed normally. I have no idea what happens with other SCSI transports. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
