Re: [PATCH] usb gadget: remove size limitation for storage cdrom

Alan Stern
2015-03-08T15:30:01Z

On Sun, 8 Mar 2015, Dave Young wrote:

> I used usb cdrom emulation to play video dvd for my daughter, but I got below
> error:
>
> [dave@darkstar tmp]$ cat /mnt/sr1/VIDEO_TS/VTS_01_5.VOB >/dev/null
> cat: /mnt/sr1/VIDEO_TS/VTS_01_5.VOB: Input/output error
> [dave@darkstar tmp]$ dmesg|tail -1
> [ 3349.371857] sr1: rw=0, want=8028824, limit=4607996
>
> The assumption of cdrom size is not right, we can create data dvd large then
> 4G, also mkisofs can create an iso with only one blank directory, the size is
> less than 300 sectors. The size limit does not make sense, thus remove them.
>
> Signed-off-by: Dave Young <dyoung@redhat.com>
> ---
> drivers/usb/gadget/function/storage_common.c | 9 ---------
> 1 file changed, 9 deletions(-)
>
> --- linux.orig/drivers/usb/gadget/function/storage_common.c
> +++ linux/drivers/usb/gadget/function/storage_common.c
> @@ -247,15 +247,6 @@ int fsg_lun_open(struct fsg_lun *curlun,
>
> num_sectors = size >> blkbits; /* File size in logic-block-size blocks */
> min_sectors = 1;
> - if (curlun->cdrom) {
> - min_sectors = 300; /* Smallest track is 300 frames */
> - if (num_sectors >= 256*60*75) {
> - num_sectors = 256*60*75 - 1;
> - LINFO(curlun, "file too big: %s\n", filename);
> - LINFO(curlun, "using only first %d blocks\n",
> - (int) num_sectors);
> - }
> - }
> if (num_sectors < min_sectors) {
> LINFO(curlun, "file too small: %s\n", filename);
> rc = -ETOOSMALL;

NAK. This patch is wrong, for a very simple reason. You wrote:

> I used usb cdrom emulation to play video dvd for my daughter

and this demonstrates the error quite plainly. You can't use _CD_
emulation to imitate a _DVD_ -- they are different sorts of media.
Just think of what happens when you try playing a DVD on a CD player.

A more suitable approach would be to add DVD emulation to the driver.

Alan Stern