On Wed, 8 Jun 2011 13:05:57 -0700 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > Perhaps, but this sounds like the reverse of what happens today where > scsi device drivers with knowledge of their own hardware will tell the > midlayer/subsystem the restriction. The change with regard to this > patch is that the scsi device driver (for example) will recognize that > the device it is driving will not be a bus master and will arrange to > allocate a dma channel from dmaengine. When said scsi driver reports > the dma restrictions to the subsystem it will borrow the parameters > from the dma channel, not the scsi device. So yes, I still think it > should be whatever the dma channel says. > > Although, you've been doing scsi work longer than I, so maybe I'm > overlooking something...? > > Are there any cases today where the subsystem imposes tighter > restrictions on the dma geometry than what the device reports? Even Yeah, there are already lots. SCSI-mid layer set the max sg entries to 128 on many architectures (see SCSI_MAX_SG_CHAIN_SEGMENTS). SCSI HBA drivers stores the max sg entries in scsi_host structure. SCSI-mid layer sets the tighter one (see __scsi_alloc_queue). blk_queue_max_segments(q, min_t(unsigned short, shost->sg_tablesize, SCSI_MAX_SG_CHAIN_SEGMENTS)); As you know, modern SCSI HBA can handle more sg entries than 128. > if that were the case it would be same situation that the scsi device > driver reports maximum parameters, but the subsystem opts for > something tighter. Whether the maximal parameters come from the scsi > device or the dma channel is moot. If only you convert all the SCSI HBA drivers to store the limit of sg entries in struct device_dma_parameters and use the proposed API. I can't find any description in this patchset about how we will convert software that already set the limit of max sg entries in a different way. I don't think that this patchset needs to convert the SCSI (and other software layers setting the limit of max sg entries). But I think that we need a new rule, the data structure, and the API about how and where everyone sets the dma restrictions and tells them to the upper software layers. -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
