linux-kernel - Re: [PATCH v2] usb: gadget: f_mass_storage: Make CD-ROM emulation works with Windows OS

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Message-ID: <YrXJss4cTtfenu90@rowland.harvard.edu>
Date:   Fri, 24 Jun 2022 10:26:58 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Neal Liu <neal_liu@...eedtech.com>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Roger Quadros <roger.quadros@...ia.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Wesley Cheng <quic_wcheng@...cinc.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Matthew Wilcox <willy@...radead.org>,
        Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        Cai Huoqing <cai.huoqing@...ux.dev>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, BMC-SW@...eedtech.com
Subject: Re: [PATCH v2] usb: gadget: f_mass_storage: Make CD-ROM emulation
 works with Windows OS

On Fri, Jun 24, 2022 at 10:19:16AM +0800, Neal Liu wrote:
> Add read TOC with format 1 to support CD-ROM emulation with
> Windows OS.
> This patch is tested on Windows OS Server 2019.
> 
> Signed-off-by: Neal Liu <neal_liu@...eedtech.com>
> ---
>  drivers/usb/gadget/function/f_mass_storage.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index 3a77bca0ebe1..9edf76c22605 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -1209,7 +1209,8 @@ static int do_read_toc(struct fsg_common *common, struct fsg_buffhd *bh)
>  
>  	switch (format) {
>  	case 0:
> -		/* Formatted TOC */
> +	case 1:
> +		/* Formatted TOC, Session info */

This comment is a little misleading.  0 is formatted TOC, and 1 is 
multi-session info.  The way you wrote it, it looks like the comment 
applies to both formats.  Do it like this:

	case 0:		/* Formatted TOC */
	case 1:		/* Multi-session info */

>  		len = 4 + 2*8;		/* 4 byte header + 2 descriptors */
>  		memset(buf, 0, len);
>  		buf[1] = len - 2;	/* TOC Length excludes length field */

It looks like you ignored one of the problems I pointed out in my 
earlier email:

> When format is 1, the driver is supposed to ignore the start_track
> value.  Your patch does not do this.

Please fix this.

Alan Stern

> @@ -1250,7 +1251,7 @@ static int do_read_toc(struct fsg_common *common, struct fsg_buffhd *bh)
>  		return len;
>  
>  	default:
> -		/* Multi-session, PMA, ATIP, CD-TEXT not supported/required */
> +		/* PMA, ATIP, CD-TEXT not supported/required */
>  		curlun->sense_data = SS_INVALID_FIELD_IN_CDB;
>  		return -EINVAL;
>  	}
> -- 
> 2.25.1
>