Re: [PATCH v3] scsi: ufs: Increase the maximum data buffer size

[Bean Huo <huobean@gmail.com>]

On Tue, 2022-09-06 at 11:04 -0700, Bart Van Assche wrote:
> 
> Hi Bean,
> 
> It seems to me that the block layer only uses the optimal transfer
> size 
> (io_opt) to determine how much data to read ahead during sequential 
> reads? See also disk_update_readahead().
> 

Bart,
Sorry for later reply, Didn't notice a question here. 

In the upstream standard Linux kernel, if the device supports a valid
VPD, I think, your this change will also not change the read-ahead
window.

....

sdkp->opt_xfer_blocks = get_unaligned_be32(&vpd->data[12]);


sd_revalidate_disk() {

...
 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);
 }	
...
}


device_add_disk()
	 disk_update_readahead(disk) {
		disk->bdi->ra_pages =
		max(queue_io_opt(q) * 2 / PAGE_SIZE,
VM_READAHEAD_PAGES); 
	}

e.g., if device reports opt 512KB, and the maximum transfer length will
be 512KB, and readahead window will be 1MB.


Kind regards,
Bean