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

[Dave Young]

On 03/09/15 at 10:22am, Alan Stern wrote:
> On Mon, 9 Mar 2015, Dave Young wrote:
> 
> > On 03/08/15 at 11:29am, Alan Stern wrote:
> > > 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@xxxxxxxxxx>
> > > > ---
> > > >  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.
> > > 
> > 
> > They are both iso9660 images, aren't they? So from data image point
> > of view there's no difference, it is not worth to create another mode
> > for dvd data.
> 
> There's more to emulation than just the image.  We have to emulate the 
> hardware's response to all the applicable commands.  CD players have a 
> different command set from DVD players (or at least, different from DVD 
> players when a DVD is inserted).  For example, see the definition in 
> the MSF format for READ TOC command.

For data cd it works without those, but yes I will take a look how can
make MSF format ok. Possiblly it will need minimum commandset support.

As for audio cd/dvd, problem is there's no standard cd media format as a file
in filesystem. Meanwhile there's no much gain for the effort.

For dvd video, mplayer works happliy with size-limit-only fix though I can
not change subtile etc, I think full support dvd video need add extra scsi
command in code.

Long long ago I ever wrote a virtual cd driver as a block driver, but I lost the code.
Will see if I can slowly rewrite one.

> 
> > We should not emulate cd drive to support different cd media type,
> > it is far more better to support just iso9660 volume no matter how
> > large the size is as long as it is fit for iso9660 spec.
> > 
> > OTOH, the size limitation is a bug:
> > * isofs can be less than 300 sectors, the 300 sectors limitation is for
> > music cd I think. Try mkisofs a blank directory and burn it.
> 
> You're thinking about this the wrong way.  We aren't emulating an 
> iso9660 image; we are emulating a CD player.  (In principle we could 
> even emulate an audio CD, but the driver doesn't support that.)

Right, but data iso is the most of use cases. For virtual cd drive we can
choose to support data cd only or data cd first.

> 
> > * There's 99 minutes dics even for cdrom, see:
> > http://en.wikipedia.org/wiki/CD-R 
> > If we code to support different size discs, it will looks like a wrong way.
> 
> Look at the code you want to remove:
> 
> > > > -		if (num_sectors >= 256*60*75) {
> 
> That's 75 sectors/second * 60 seconds/minute * 256 minutes.  So the
> driver already supports up to 256 minutes, which is way beyond the 99
> minutes required by the spec.

Oops, I missed the line, so as I said I will take a look at how to fix it
both msf and *lba* mode.

But I have limited time, will response slow and play with it at weekend.

Thanks
Dave
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