Re: [PATCH v2 0/7] Rename scsi_get_lba() into scsi_get_pos()

From: Bart Van Assche <bvanassche@acm.org>
To: jejb@linux.ibm.com, "Martin K . Petersen" <martin.petersen@oracle.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>, Bean Huo <beanhuo@micron.com>,
	Avri Altman <avri.altman@wdc.com>,
	Asutosh Das <asutoshd@codeaurora.org>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Can Guo <cang@codeaurora.org>,
	linux-scsi@vger.kernel.org
Subject: Re: [PATCH v2 0/7] Rename scsi_get_lba() into scsi_get_pos()
Date: Wed, 12 May 2021 15:20:47 -0700	[thread overview]
Message-ID: <b27a3c7d-1c10-faaa-4c33-273a463faa80@acm.org> (raw)
In-Reply-To: <96a253f8776a7736b480bdf190840440ffb4e53c.camel@linux.vnet.ibm.com>

On 5/12/21 3:10 PM, James Bottomley wrote:
> On Wed, 2021-05-12 at 13:08 -0700, Bart Van Assche wrote:
>> This patch series renames scsi_get_lba() into scsi_get_pos(). The
>> name of scsi_get_lba() is confusing since it does not return an LBA
>> but instead the start offset divided by 512.
> 
> OK, I'll bite: given the logical block size for all drives is 512 why
> is logical block address not the start offset in bytes divided by 512?

My understanding is that LBA = logical block address = (start offset in 
bytes) / (logical block size) and also that the Linux kernel supports 
logical block sizes between 512 bytes and 4 KiB. From drivers/scsi/sd.c 
(sector_size represents the logical block size):

	if (sector_size != 512 &&
	    sector_size != 1024 &&
	    sector_size != 2048 &&
	    sector_size != 4096) {
		sd_printk(KERN_NOTICE, sdkp,
			 "Unsupported sector size %d.\n",
			  sector_size);
		/*
		 * The user might want to re-format the drive with
		 * a supported sectorsize.  Once this happens, it
		 * would be relatively trivial to set the thing up.
		 * For this reason, we leave the thing in the table.
		 */
		sdkp->capacity = 0;
		/*
		 * set a bogus sector size so the normal read/write
		 * logic in the block layer will eventually refuse any
		 * request on this device without tripping over power
		 * of two sector size assumptions
		 */
		sector_size = 512;
	}

Thanks,

Bart.