Alan Stern wrote: > This patch (as857) modifies the SG_GET_RESERVED_SIZE and > SG_SET_RESERVED_SIZE ioctls in the sg driver, capping the values at > the device's request_queue's max_sectors value. This will permit > cdrecord to obtain a legal value for the maximum transfer length, > fixing Bugzilla #7026. > > The patch also caps the initial reserved_size value. There's no > reason to have a reserved buffer larger than max_sectors, since it > would be impossible to use the extra space. > > The corresponding ioctls in the block layer are modified similarly, > and the initial value for the reserved_size is set as large as > possible. This will effectively make it default to max_sectors. > Note that the actual value is meaningless anyway, since block devices > don't have a reserved buffer. > > Finally, the BLKSECTGET ioctl is added to sg, so that there will be a > uniform way for users to determine the actual max_sectors value for > any raw SCSI transport. I think you actually want max_hw_sectors. Well, you might and you might not :) I think a estimate of the max transfer length would be more like: unsigned int max_segment_size, max_xfer, int max_segments; /* * does not account for any weird arch segments boundary limits * or vmerge limits */ if (!(q->queue_flags & (1 << QUEUE_FLAG_CLUSTER))) max_segment_size = PAGE_SIZE; else max_segment_size = q->max_segment_size; max_segments = min(q->max_hw_segments, q->max_phys_segments); max_xfer = min(q->max_hw_sectors * 512, max_segments * max_segment_size); return max_xfer; The problem is that we assume we will get nice large segments. When using sg it will try to allocate multiple pages and make large segments. We could hit a bad case where we cannot allocate enough large segments, so a worst case would result in a max_segment_size of PAGE_SIZE: max_segments = min(q->max_hw_segments, q->max_phys_segments); max_xfer = min(q->max_hw_sectors * 512, max_segments * PAGE_SIZE); return max_xfer; I think you have to take into account the scatterlist limits because some drivers like lpfc have a small sht->sg_tablesize/q->max_hw_segments (64), but have a large q->max_hw_sectors (0xFFFF) and uses the default q->max_sectors (1024). So in the worst case we could end up with a max transfer size of only 64 * PAGE_SIZE and this a lot smaller than q->max_sectors. - 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
