В Sun, 10 Apr 2022 11:21:15 -0400
Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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@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.
>
> 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@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?
>
> 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!
