Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sun, 10 Apr 2022 05:18:04 +0300
From:   Maxim Devaev <mdevaev@gmail.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     linux-usb@vger.kernel.org, Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Cai Huoqing <caihuoqing@baidu.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] usb: gadget: f_mass_storage: break IO operations via
 configfs
Message-ID: <20220410051804.315fb0d9@reki>
In-Reply-To: <YlI5b1SNdsVnuo/v@rowland.harvard.edu>
References: <Yk3TLPKyaQDsnuD4@rowland.harvard.edu>
        <20220406213634.104cae45@reki>
        <Yk8L6b9wEWTjWOg4@rowland.harvard.edu>
        <20220407204553.35cead72@reki>
        <YlBN4Zcn9NYw0PLA@rowland.harvard.edu>
        <20220409115756.4f9b015d@reki>
        <YlGOOJ9SwkD7WVmX@rowland.harvard.edu>
        <20220409170837.02f0853f@reki>
        <YlHrBdYkBSR0L6FL@rowland.harvard.edu>
        <20220410014228.117d9f66@reki>
        <YlI5b1SNdsVnuo/v@rowland.harvard.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

=D0=92 Sat, 9 Apr 2022 21:57:03 -0400
Alan Stern <stern@rowland.harvard.edu> =D0=BF=D0=B8=D1=88=D0=B5=D1=82:

> On Sun, Apr 10, 2022 at 01:42:28AM +0300, Maxim Devaev wrote:
> > =D0=92 Sat, 9 Apr 2022 16:22:29 -0400
> > Alan Stern <stern@rowland.harvard.edu> wrote: =20
> > > > I'm using Raspberry Pi with DWC2. So:
> > > > - Connect RPi-based gadget to the Linux host.
> > > > - Set image in the "file" attribute.   =20
> > >=20
> > > Exactly what is the full pathname you're using for the "file" attribu=
te? =20
> >=20
> > /sys/kernel/config/usb_gadget/kvmd/functions/mass_storage.usb0/lun.0/fi=
le =20
>=20
> Yeah, that doesn't seem right at all.
>=20
> You're doing this under KVM, right?  Is the gadget driver running in the=
=20
> host OS or the guest OS?  And the sysfs file accesses -- are they in the=
=20
> host's filesystem or in the guest's?
>=20
> What happens if you don't use KVM and just load the gadget driver on the=
=20
> 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 devic=
es:
with USB host and USB gadget.

> > > I also tried sending a USR1 signal to the driver's kernel thread whil=
e=20
> > > an image was mounted and being accessed.  It did clear the prevent_al=
low=20
> > > flag, so I could eject the image.  But it also caused a 30-second del=
ay=20
> > > on the host, as predicted.  Now, maybe you don't care about such dela=
ys=20
> > > when you're going to eject the media anyway, but it still seems like =
a=20
> > > bad thing to do. =20
> >=20
> > It looks like the prevent_medium_removal flag switching really works be=
tter in this case. =20
>=20
> 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_rem=
oval=3D0
and makes the "file" empty.

>=20
> > > > > > I have reflected on the rest of your arguments and changed my m=
ind.
> > > > > > I think that "forced_eject" for a specific lun without interrup=
ting 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?
> > > > > >=20
> > > > > >=20
> > > > > > static ssize_t fsg_lun_opts_forced_eject_store(struct config_it=
em *item,
> > > > > >                                                const char *page=
, size_t len)
> > > > > > {
> > > > > >         struct fsg_lun_opts *opts =3D to_fsg_lun_opts(item);
> > > > > >         struct fsg_opts *fsg_opts =3D to_fsg_opts(opts->group.c=
g_item.ci_parent);
> > > > > >         int ret;
> > > > > >=20
> > > > > >         opts->lun->prevent_medium_removal =3D 0;
> > > > > >         ret =3D fsg_store_file(opts->lun, &fsg_opts->common->fi=
lesem, "", 0);
> > > > > >         return ret < 0 ? ret : len;
> > > > > > }
> > > > > >=20
> > > > > > CONFIGFS_ATTR_WO(fsg_lun_opts_, forced_eject);     =20
> > > > >=20
> > > > > The basic idea is right.  But this should not be a CONFIGFS optio=
n; it=20
> > > > > should be an ordinary LUN attribute.  For an example, see the def=
inition of=20
> > > > > file_store() in f_mass_storage.c; your routine should look very s=
imilar.   =20
> > > >=20
> > > > Okay, but where this attribute is located in sysfs? How can I use i=
t?   =20
> > >=20
> > > Well, it's going to be in different places depending on what UDC driv=
er=20
> > > your gadget uses.  On my system I'm using the dummy_udc driver, so th=
e=20
> > > sysfs "file" attribute is located at:
> > >=20
> > > 	/sys/devices/platform/dummy_ucd.0/gadget/lun0/file
> > >=20
> > > If instead you're looking at
> > >=20
> > > 	/sys/module/g_mass_storage/parameters/file
> > >=20
> > > or in some configfs directory, that's the wrong place.  You can eject=
=20
> > > the media simply by doing (as root):
> > >=20
> > > 	echo >/sys/devices/.../gadget/lun0/file
> > >=20
> > > (fill in the "..." appropriately for your system).
> > >  =20
> > > > Sorry for the stupid question.   =20
> > >=20
> > > Not at all. =20
> >=20
> > Thanks! Unfortunately I'm using dwc2 driver and it doesn't have any gad=
get parameters
> > outside of the configfs:
> >=20
> > [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 ~]#
> >=20
> > So in my local case configfs is only way to place forced_eject :( =20
>=20
> That can't possibly be right.  Again, we may be miscommunicating because=
=20
> of the way you're using KVM.
>=20
> What happens if you set up the gadget using g-mass-storage instead of=20
> configfs?  For example:
>=20
> 	modprobe g-mass-storage cdrom=3Dy removable=3Dy ro=3Dy file=3D...
>
> > Could we add both device attrs and configfs file? =20
>=20
> No.  Configfs files are for setting up the gadget in the first place, or=
=20
> changing its configuration while it isn't attached to a host.  Device=20
> attribute files are for modifying the gadget while it is running.
>=20
I've tried and got this:

[root@pikvm ~]# modprobe g-mass-storage cdrom=3Dy removable=3Dy ro=3Dy file=
=3D/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 co=
nfigfs
and device attributes is almost identical and everything worked.
If I don't have device attributes when using libcomposite, then how am I su=
pposed
to change its settings in runtime, if not through configfs?