Re: [PATCH 1/1] scsi: sd: use max_xfer_blocks for set rw_max if max_xfer_blocks is available

[Changheun Lee <nanich.lee@samsung.com>]

> Damien,
> 
> >> How about set larger valid value between sdkp->max_xfer_blocks,
> >> and sdkp->opt_xfer_blocks to rw_max?
> >
> > Again, if your device reports an opt_xfer_blocks value that is too
> > small for its own good, that is a problem with this device.
> 
> Correct. It is very much intentional that we do not default to issuing
> the largest commands supported by the physical hardware.
> 
> If the device is not reporting an optimal transfer size, and the block
> layer default is too small, the solution is to adjust max_sectors_kb in
> sysfs (by adding a udev rule, for instance).
>

Sorry, I said wrong.
As others mentioned, it's device problem if opt_xfer_blocks is wrong.
kernel modification is not needed for it.

I want to discuss using max_xfer_blocks instead of opt_xfer_blocks as a optional.
For example, device reports opt_xfer_blocks is 512KB and 1MB as a
max_xfer_blocks too. Currently rw_max is set with 512KB only.
I want a option to select max_xfer_blocks for rw_max.
Are there any historical problem when rw_max is set with max_xfer_blocks?

How about below approach if max_xfer_blocks can be set to rw_max?
It's using queue flag as a option. Do you have good idea to suggest?

static bool sd_validate_max_xfer_size(struct scsi_disk *sdkp,
				      unsigned int dev_max)
{
	struct request_queue *q = sdkp->disk->queue;

	if (!blk_queue_allow_sd_rw_max(q))
		return false;

	if (sdkp->max_xfer_blocks == 0)
		return false;

        ......

	return true;
}

static int sd_revalidate_disk(struct gendisk *disk)
{
	......

	if (sd_validate_max_xfer_size(sdkp, dev_max)) {
		q->limits.io_opt = logical_to_bytes(sdp, sdkp->max_xfer_blocks);
		rw_max = logical_to_sectors(sdp, sdkp->max_xfer_blocks);
	} else if (sd_validate_opt_xfer_size(sdkp, dev_max)) {
		q->limits.io_opt = logical_to_bytes(sdp, sdkp->opt_xfer_blocks);
		rw_max = logical_to_sectors(sdp, sdkp->opt_xfer_blocks);
	} else {
	
	......
}

> -- 
> Martin K. Petersen	Oracle Linux Engineering