Hello.

I have a digital camera that supports mass storage ; I use a kernel 2.4.21 from kernel.org (no patchs).

My camera is recognized as a usb peripheral (I can see it with lsusb) but a scsi device should be created, it is not. Note that I included SCSI emulation in my kernel.

My distribution is a Gentoo - how could I make my camera work ?

unless you're using devfs, the scsi device won't be created. You'll have to create it ahead of time, and when the camera is recognized and setup as a scsi drive, it will be bound to the scsi device.
When you say "isn't created" I'm assuming that you mean /dev/scd? doesn't exist

Maybe some of these are useful:

USB Mass Storage Devices (Compact Flash Card Readers) in Linux
Mass Storage Devices
Linux USB Support for Casio QV 3/2x00/3x00/4000/8000 Digital Still Cameras

Take a look at :
http://gphoto.sourceforge.net/

If you have this program on your computer, use "gphoto2 --list-cameras" to get an overview of supported camera's. Some camera's might be accessible as an USB device.

What does "dmesg |tail" say _after_ you've plugged in your USB camera?

gphoto does not support my camera (a Kyocera S3x)

From dmesg:

hub.c: new USB device 00:1d.0-1.3, assigned address 5
usb-storage: act_altsettting is 0
usb-storage: id_index calculated to be: 59
usb-storage: Array length appears to be: 74
usb-storage: USB Mass Storage device detected
usb-storage: Endpoints: In: 0xd42ae6b4 Out: 0xd42ae6a0 Int: 0xd42ae6c8 (Period 255)
usb-storage: Found existing GUID 048201000000000140010711
WARNING: USB Mass Storage data integrity not assured
USB Mass Storage device found at 5

Yes, I mean that /dev/scd? does not exist. I have a /dev/scsi empty directory. Actually I'm using devfs, and it properly create the device for my external dvd drive (I have a laptop) - at least when plugged before booting.

Maybe my problem comes from hotplug...

True USB mass storage devices act like removable SCSI harddisks, and as such would be under /dev/sdX rather than /dev/scdX. Plug in your camera and type cdrecord -scanbus. That will output any and all devices that are on your scsi buses (ide or usb, or real scsi). If you see mention of your device, then it has been recognised - this does not mean, however, that you can definitely access your photos. If you have no other scsi disk devices (no pendrives, no zip drive, nothing) then you should notice /dev/sda and most likely /dev/sda1 (if not, then as root run fdisk -l /dev/sda presuming sda exists). If you're this far and all is well, you can try mounting it mount -t vfat /dev/sdXX /mnt/camera (make sure you have already made the directory /mnt/camera). Voila.

I stated above that even though things might look rosey, they can go wrong. I recently bought a nice digital camera which was recognised as a USB mass storage, and indeed the /dev/sda entry was there... however, the camera didn't allow access to the partitions, so I still couldn't get to the photos. A few quid/bucks later, I had a USB card reader and all is well.