On Tue, 2009-05-12 at 09:19 +0900, Tejun Heo wrote: > Hello, all. > > James Bottomley wrote: > >>> Does resid_len make any sense w/ failed requests? I think we would be > >>> better off with declaring residual count to be undefined on request > >>> failure. Is there any place which depends on it? > >> IIRC, I wrote the code. I think that this doesn't matter but it's > >> better not to change the behavior unless Eric ack on this change > >> (maybe LSI has some management binary that assume this behavior though > >> it's unlikely). > > > > Actually, yes it does, for many possible reasons. > > > > The first being if the device is too stupid to report an actual sector > > location the next best way of determining where the error occurred is > > from the residual. We don't make use of this in kernel (perhaps we > > should?) but some of the user space programs for CD/DVD burning do. > > Really? Residual count on command success is used but on failure? > That's a dangerous territory. When a SG_IO fails, the only data the > app should be accessing is the sense data if the status indicates its > validity. The problems with residual count on failed command are... > > * Not well defined. What does it mean really? It can't indicate > successful partial transfer. If the request partially succeeded, > the required behavior is to successfully complete the request > partially with residual count and then fail the latter part when > issued again. If the failure applies to the whole request but > location information is useful, it should be carried in the sense > data. The definition is the amount of data transfer requested less the actual that went over the wire ... that's certainly a well defined quantity; although, one could argue about what this means in the device. Certainly I agree that just because the data was transferred to or from the device is no guarantee that the device did anything with it (or transferred it accurately). > * What about corner values? What does 0 or full resid count on > failure mean? 0 means everything transferred, full residual means nothing did. > * Different layers of failing. In SG_IO interface, a request may fail > with -EIO way before it reaches block layer. Residual count can't > be set to any meaningful value in these cases. We can set it to > full count for these fast fail paths, but do we really wanna go > there? Another problem is when a driver is missing SG_IO > capability. Who's responsible for setting resid count in that case? > How is upper layer gonna determine a SG_IO failed because lower > level driver didn't support it or it genuinely failed? Well, I prefer the concept of transfer length, which would be initialised to zero ... however, residuals should be initialised to the actual transfer count. > I think it's just silly to give any meaning to resid count when the > request fails. It's best to leave the field unmodified or just > declare it undefined. It's current behaviour. Technically that makes it part of the SG_IO ABI ... although it could be deprecated if someone can verify there are no current users. James -- 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
