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

[Maxim Devaev]

В Sat, 9 Apr 2022 21:57:03 -0400
Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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.

> > > 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.

> 
> > > > > > 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@pikvm ~]# find /sys -iname lun0
> > [root@pikvm ~]# find /sys -iname lun.0
> > /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0
> > [root@pikvm ~]#
> > 
> > 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@pikvm ~]# modprobe g-mass-storage cdrom=y removable=y ro=y file=/var/lib/kvmd/msd/images/dsl-4.11.rc1.iso
[root@pikvm ~]# find /sys -iname lun.0
[root@pikvm ~]# find /sys -iname lun0
/sys/devices/platform/soc/fe980000.usb/gadget/lun0
[root@pikvm ~]# 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@pikvm ~]# 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?