On Fri, 12 Aug 2011, Barry Song wrote:
> From: Peiyu Li <peiyu.li@xxxxxxx>
>
> Now the mass storage driver has fixed logic block size of 512 bytes.
>
> The mass storage gadget read/write bound devices only through vfs, so the bottom level
> devices actually are just RAW devices to the driver and connected PC. As a raw, hosts
> can always format, read and write it right in 512 bytes logic block and don't care about
> the actual logic block devices bound to the gadget.
>
> But if we want to share the bound block device partition between target board and PC,
> in case the logic block size of the bound block device is 4KB, we execute the following
> steps:
> 1. connect a board with mass storage gadget to PC(the board has set one partition of
> on-board block device as file name of the mass storage)
> 2. PC format the mass storage to VFAT by default logic block size and read/write it
> 3. disconnect boards from PC
> 4. target board mount the partition as VFAT
> Step 4 will fail since kernel on target thinks the logic block size of the bound partition
> as 4KB.
> A typical error is
> "FAT: logical sector size too small for device (logical sector size = 512)"
>
> If we execute opposite steps:
> 1. format the partion to FAT on target board and read/write this partition
> 2. connect the board to Windows PC as usb mass storage gadget, windows will think the disk
> is not formatted
>
> So the conclusion is that only as a gadget, the mass storage driver has no any problem.
> But being shared VFAT or other filesystem on PC and target board, it will fail.
>
> This patch adapts logic block size to bound block devices and fix the issue.
...
> @@ -580,7 +581,9 @@ static int fsg_lun_open(struct fsg_lun *curlun, const char *filename)
> rc = (int) size;
> goto out;
> }
> - num_sectors = size >> 9; /* File size in 512-byte blocks */
> + curlun->l_blkbits = blksize_bits(
> + bdev_logical_block_size(inode->i_bdev));
Will this work if the backing file is not a block device? What if it's
just a regular file?
As long as you're doing this, you might as well store (1 << l_blkbits)
in curlun too, instead of recomputing it all the time.
> + num_sectors = size >> curlun->l_blkbits; /* File size in logic-block-size blocks */
> min_sectors = 1;
> if (curlun->cdrom) {
> num_sectors &= ~3; /* Reduce to a multiple of 2048 */
And as long as you're using variable logical block sizes, you might as
well set l_blkbits here to 11, because cdroms always use a block size
of 2048.
Alan Stern
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