Raspberry Pi Zero as read write mass storage - module

Question

I am trying to setup a Raspberry Pi Zero as a mass storage with dwc2 and g_mass_storage (using the last Raspbian image available).
I created the data storage file with dd, made it a FAT32 fs with mkdosfs.
I looked for a lot of things. I spent some time to understand that options should be passed in /etc/modprobe.d/g_mass_storage.conf
It is finally seen in ubuntu. My problem is : it is in read only mode.
I tried to set the ro option to y or n without any impact on the behaviour. I changed permissions on the file (777), it changed nothing.
here is the current content of my /etc/modprobe.d/g_mass_storage.conf file :
options g_mass_storage file=/piusb.bin stall=0 removable=y ro=n
here is the of dmesg from the ubuntu :
[Today and now]usb 1-1.2: new high-speed USB device number 28 using ehci-pci
[ +0,299994] usb 1-1.2: device descriptor read/64, error -71
[ +0,959969] usb 1-1.2: device descriptor read/64, error -71
[ +0,187918] usb 1-1.2: new high-speed USB device number 29 using ehci-pci
[ +0,113925] usb 1-1.2: New USB device found, idVendor=0525, idProduct=a4a5
[ +0,000007] usb 1-1.2: New USB device strings: Mfr=3, Product=4, SerialNumber=0
[ +0,000004] usb 1-1.2: Product: Mass Storage Gadget
[ +0,000003] usb 1-1.2: Manufacturer: Linux 4.19.118+ with 20980000.usb
[ +0,001467] usb-storage 1-1.2:1.0: USB Mass Storage device detected
[ +0,000246] usb-storage 1-1.2:1.0: Quirks match for vid 0525 pid a4a5: 10000
[ +0,000283] scsi host9: usb-storage 1-1.2:1.0
[ +1,009528] scsi 9:0:0:0: Direct-Access Linux File-Stor Gadget 0419 PQ: 0 ANSI: 2
[ +0,001079] sd 9:0:0:0: Attached scsi generic sg5 type 0
[ +0,001699] sd 9:0:0:0: Power-on or device reset occurred
[ +0,001211] sd 9:0:0:0: [sde] 67108864 512-byte logical blocks: (34.4 GB/32.0 GiB)
[ +0,000672] sd 9:0:0:0: [sde] Write Protect is on
[ +0,000016] sd 9:0:0:0: [sde] Mode Sense: 0f 00 80 00
[ +0,000740] sd 9:0:0:0: [sde] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ +0,005478] sde:
[ +0,002735] sd 9:0:0:0: [sde] Attached SCSI removable disk
What to do to get a read/write mass storage ?

Answer 1

I reproduced your problem. I had a similar problem with the module g_acm_ms on Raspberry Pi Zero W. If you configure the module in /etc/modprobe.d/, then the system ignores the parameter ro=0. Perhaps this is due to the order of initialization of kernel modules. I got around this by adding the module to the blacklist and writing the initialization in /etc/rc.local. Thus, I managed to get an emulation of a read/write mass storage.
All steps:
Create and format the file:
# dd bs=1M if=/dev/zero of="/piusb.bin" count=2048
# mkdosfs "/piusb.bin"
Add modules-load=dwc2,g_acm_ms to end of /boot/cmdline.txt (after rootwait)
Add the string dtoverlay=dwc2 in /boot/config.txt
Prevent module initialization at startup:
$ echo "blacklist g_acm_ms" | sudo tee -a "/etc/modprobe.d/blacklist-g_acm_ms.conf"
Add initialization in /etc/rc.local above exit 0:
# nano /etc/rc.local
Add string:
/sbin/modprobe file=/piusb.bin removable=y ro=0 stall=0
Reboot system:
# reboot -h
P.S.: If you don't want to use USB for console output, you can use g_mass_storage instead of g_acm_ms.