linux-kernel - Re: [PATCH] usb: gadget: f_mass_storage: break IO operations via configfs

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Date:   Sat, 9 Apr 2022 17:08:37 +0300
From:   Maxim Devaev <mdevaev@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     linux-usb@...r.kernel.org, Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cai Huoqing <caihuoqing@...du.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: gadget: f_mass_storage: break IO operations via
 configfs

В Sat, 9 Apr 2022 09:46:32 -0400
Alan Stern <stern@...land.harvard.edu> wrote:

> On Sat, Apr 09, 2022 at 11:57:56AM +0300, Maxim Devaev wrote:
> > В Fri, 8 Apr 2022 10:59:45 -0400
> > Alan Stern <stern@...land.harvard.edu> wrote:  
> > > > At least there is one situation where the behavior of f_mass_storage differs
> > > > from the behavior of a real drive. What happens when you click on the physical
> > > > "eject" button?    
> > > 
> > > If the host has prevented ejection, nothing happens.  Otherwise the disc 
> > > gets ejected.
> > >   
> > > > Yes, the OS can block this, but the problem is that we don't have
> > > > an "eject" here.    
> > > 
> > > What do you mean?  Writing an empty string to the sysfs "file" attribute 
> > > is the virtual analog of pressing the eject button.  
> > 
> > But I can't eject the disc event it's not mounted on Linux host. It seems to me
> > it differs from the real drive behavior.  
> 
> It sounds like either there's a bug or else you're not doing the right 
> thing.  Tell me exactly what you do when this fails.

I'm using Raspberry Pi with DWC2. So:
- Connect RPi-based gadget to the Linux host.
- Set image in the "file" attribute.
- Mount gadget's drive on the Linux host.
- Umount it.
- Try to eject using emptying the "file" attribute.
- Get EBUSY error.

> 
> > > ...  
> > 
> > I have reflected on the rest of your arguments and changed my mind.
> > I think that "forced_eject" for a specific lun without interrupting operations would
> > really be the best solution. I wrote a simple patch and tested it, everything seems
> > to work. What do you think about something like this?
> > 
> > 
> > static ssize_t fsg_lun_opts_forced_eject_store(struct config_item *item,
> >                                                const char *page, size_t len)
> > {
> >         struct fsg_lun_opts *opts = to_fsg_lun_opts(item);
> >         struct fsg_opts *fsg_opts = to_fsg_opts(opts->group.cg_item.ci_parent);
> >         int ret;
> > 
> >         opts->lun->prevent_medium_removal = 0;
> >         ret = fsg_store_file(opts->lun, &fsg_opts->common->filesem, "", 0);
> >         return ret < 0 ? ret : len;
> > }
> > 
> > CONFIGFS_ATTR_WO(fsg_lun_opts_, forced_eject);  
> 
> The basic idea is right.  But this should not be a CONFIGFS option; it 
> should be an ordinary LUN attribute.  For an example, see the definition of 
> file_store() in f_mass_storage.c; your routine should look very similar.

Okay, but where this attribute is located in sysfs? How can I use it?
Sorry for the stupid question.