linux-kernel - Re: [PATCH v2 2/3] usb: gadget: file_storage: Make CD-ROM emulation work with Mac OS-X

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Date:	Tue, 22 Mar 2011 10:26:34 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Roger Quadros <roger.quadros@...ia.com>
cc:	gregkh@...e.de, <sshtylyov@...sta.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] usb: gadget: file_storage: Make CD-ROM emulation
 work with Mac OS-X

On Tue, 22 Mar 2011, Roger Quadros wrote:

> I have a question here.
> 
> The host can request us to send less or more than the actual TOC size, since it 
> has no clue how big it is.
> e.g. Linux host requests us to send only 12 bytes even though our formatted TOC 
> length is 20. In this case should we return fsg->data_size_from_cmnd instead of 
> actual TOC length?

No.  Always return the actual TOC length.

> e.g. Mac requests us to send 65534 bytes but our RAW TOC length is 37.
> The file storage driver seems to be zero padding our data response. So we 
> respond with 65534 bytes, 37 of TOC and remaining zero padded.
> 
> Can we do something like this to avoid unnecessary zero padded transfers?
> 
>          ret = fsg_get_toc(curlun, msf, format, buf);
>          if (ret < 0) {
>                  curlun->sense_data = SS_INVALID_FIELD_IN_CDB;
>                  return -EINVAL;
>          } else if (ret > fsg->data_size_from_cmnd) {
>                  ret = fsg->data_size_from_cmnd;
>          } else {
>                  fsg->residue = ret;
>          }
>          return ret;

Not needed (and not correct).  The code at the end of do_scsi_command()  
already does this:

		reply = min((u32) reply, fsg->data_size_from_cmnd);
		...
		fsg->residue -= reply;

Alan Stern

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