Date:   Sun, 10 Apr 2022 19:14:38 +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 Sun, 10 Apr 2022 11:21:15 -0400
Alan Stern <stern@...land.harvard.edu> wrote:

> On Sun, Apr 10, 2022 at 05:18:04AM +0300, Maxim Devaev wrote:
> > В Sat, 9 Apr 2022 21:57:03 -0400
> > Alan Stern <stern@...land.harvard.edu> пишет:
> >   
> > > On Sun, Apr 10, 2022 at 01:42:28AM +0300, Maxim Devaev wrote:  
> > > > В Sat, 9 Apr 2022 16:22:29 -0400
> > > > Alan Stern <stern@...land.harvard.edu> wrote:    
> > > > > > I'm using Raspberry Pi with DWC2. So:
> > > > > > - Connect RPi-based gadget to the Linux host.
> > > > > > - Set image in the "file" attribute.      
> > > > > 
> > > > > Exactly what is the full pathname you're using for the "file" attribute?    
> > > > 
> > > > /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0/file    
> > > 
> > > Yeah, that doesn't seem right at all.
> > > 
> > > You're doing this under KVM, right?  Is the gadget driver running in the 
> > > host OS or the guest OS?  And the sysfs file accesses -- are they in the 
> > > host's filesystem or in the guest's?
> > > 
> > > What happens if you don't use KVM and just load the gadget driver on the 
> > > physical machine?  
> > 
> > We really have a miscommunication :) Speaking of KVM, I mean KVM-over-IP,
> > a physical device that emulates Keyboard-Video-Mouse. It is made on the
> > Raspberry Pi and is physically connected via USB to another host machine
> > to emulate mass storage, among other things. So, we have two physical devices:
> > with USB host and USB gadget.  
> 
> Okay, I see where I misunderstood.  Oops.  :-)
> 
> > > > > I also tried sending a USR1 signal to the driver's kernel thread while 
> > > > > an image was mounted and being accessed.  It did clear the prevent_allow 
> > > > > flag, so I could eject the image.  But it also caused a 30-second delay 
> > > > > on the host, as predicted.  Now, maybe you don't care about such delays 
> > > > > when you're going to eject the media anyway, but it still seems like a 
> > > > > bad thing to do.    
> > > > 
> > > > It looks like the prevent_medium_removal flag switching really works better in this case.    
> > > 
> > > I don't understand that comment.  In what case?  Works better than what?  
> > 
> > Sorry, better than SIGUSR1. The patch that only sets the prevent_medium_removal=0
> > and makes the "file" empty.  
> 
> Ah, yes, I agree.
> 
> > > > > > > > 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?      
> > > > > 
> > > > > Well, it's going to be in different places depending on what UDC driver 
> > > > > your gadget uses.  On my system I'm using the dummy_udc driver, so the 
> > > > > sysfs "file" attribute is located at:
> > > > > 
> > > > > 	/sys/devices/platform/dummy_ucd.0/gadget/lun0/file
> > > > > 
> > > > > If instead you're looking at
> > > > > 
> > > > > 	/sys/module/g_mass_storage/parameters/file
> > > > > 
> > > > > or in some configfs directory, that's the wrong place.  You can eject 
> > > > > the media simply by doing (as root):
> > > > > 
> > > > > 	echo >/sys/devices/.../gadget/lun0/file
> > > > > 
> > > > > (fill in the "..." appropriately for your system).
> > > > >     
> > > > > > Sorry for the stupid question.      
> > > > > 
> > > > > Not at all.    
> > > > 
> > > > Thanks! Unfortunately I'm using dwc2 driver and it doesn't have any gadget parameters
> > > > outside of the configfs:
> > > > 
> > > > [root@...vm ~]# find /sys -iname lun0
> > > > [root@...vm ~]# find /sys -iname lun.0
> > > > /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0
> > > > [root@...vm ~]#
> > > > 
> > > > So in my local case configfs is only way to place forced_eject :(    
> > > 
> > > That can't possibly be right.  Again, we may be miscommunicating because 
> > > of the way you're using KVM.
> > > 
> > > What happens if you set up the gadget using g-mass-storage instead of 
> > > configfs?  For example:
> > > 
> > > 	modprobe g-mass-storage cdrom=y removable=y ro=y file=...
> > >  
> > > > Could we add both device attrs and configfs file?    
> > > 
> > > No.  Configfs files are for setting up the gadget in the first place, or 
> > > changing its configuration while it isn't attached to a host.  Device 
> > > attribute files are for modifying the gadget while it is running.
> > >   
> > I've tried and got this:
> > 
> > [root@...vm ~]# modprobe g-mass-storage cdrom=y removable=y ro=y file=/var/lib/kvmd/msd/images/dsl-4.11.rc1.iso
> > [root@...vm ~]# find /sys -iname lun.0
> > [root@...vm ~]# find /sys -iname lun0
> > /sys/devices/platform/soc/fe980000.usb/gadget/lun0
> > [root@...vm ~]# ls /sys/devices/platform/soc/fe980000.usb/gadget/lun0
> > power  file  nofua  ro  uevent
> > 
> > But with libcomposite and configfs I don't have "/sys/devices/platform/soc/fe980000.usb/gadget/lun0" at all:
> > 
> > [root@...vm ~]# ls /sys/devices/platform/soc/fe980000.usb/gadget/
> > power  suspended  uevent
> > 
> > So all this timed I used configfs to change parameters.
> > I thought this was the way it was intended because the code for changing configfs
> > and device attributes is almost identical and everything worked.
> > If I don't have device attributes when using libcomposite, then how am I supposed
> > to change its settings in runtime, if not through configfs?  
> 
> All right.  I've never used configfs before, so my understanding of it 
> was out of date.  After reading through the documentation and the code, 
> it's clear now that you're right and there should be both a device 
> attribute and a configfs file.
> 
> Unlike the fsg_lun_opts_forced_eject_store() example you wrote above, 
> but like the existing fsg_lun_opts_file_store() and file_store() 
> routines, both of your new routines should call a single 
> fsg_store_forced_eject() function in storage_common.c to do the real 
> work.  Namely, something like:
> 
> 	lun->prevent_medium_removal = 0;
> 	return fsg_store_file(lun, filesem, "", 0);
> 
> That should accomplish what you're looking for, in all possible 
> configurations.

Great! So, next I will test this patch locally and with my users and submit the v2 version
a little later. Thank you for your help!

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