gsc -- a device driver for a handy scanner
Minor number bits: uu d g p ...
uu unit gsc0 .. gsc3
d selects logging of debug messages
g selects bitmap vs. graymap output
p selects raw vs. portable pnm output
The gsc character device driver currently handles only the Genius GS-4500
handy scanner. It operates in pure DMA modes, although the hardware
could be set up to work with irq. I had neither enough documentation nor
experience in writing interrupt driven device drivers.
The device can operate at four different resolutions: 100, 200, 300 and
400dpi. It produces a simple bitmap with the most significant bit at the
left side. The driver can optionally output the famous and likely simple
portable bitmap file format pbm(5) by Jef Poskanzer. Thus the scans can
easily processed by any graphic package around ( xpaint(1), xv(1), xli(1)
only to name some of them ...). In raw mode a bit which is set means a
black pixel because the scanner detects black points on white paper. On
the other hand, because pnm format describes intensities of electron
beams in video screens a set bit in pbm mode means a white pixel.
The width of the output bitmap is fixed as given by the resolution value.
However, the height of the bitmap must be supplied in pnm mode since the
driver must know at what time the `end-of-file' shall be reached. With
this feature you are able to directly copy the scanner output into a pbm
file with cat(1). Of course you can obtain a similar effect by using
dd(1) with the driver in raw mode.
The graymap output mode is not yet implemented into the driver. It is
even questionable if external programs would not do this job better
thereby not counting to the size of the kernel. Even though, I do not
know of tools which produce a graymap from a halftone bitmap.
The ioctl requests that are served by gsc are listed below. There is a
utility, called sgsc(1), that provides access to these requests from
Set the resolution value. If this call is made after the first
read access to the device there will be no effect unless the
device is closed and opened again.
Get current resolution in dots per inch (dpi).
Set resolution value from selector switch. The driver must be in
an open though untouched state otherwise the request will fail
and errno(2) is set to EBUSY.
Set the width of the bitmap. Actually, this is an alternative
way of setting the resolution, since any allowed resolution
matches exactly one width. Allowed are listed in the table
100 dpi 424 pixels
200 dpi 840 pixels
300 dpi 1264 pixels
400 dpi 1648 pixels
? 1696 pixels
? 2544 pixels
? 3648 pixels
Values which are not reported in the above table will cause the
ioctl call to fail with errno(2) set to EINVAL.
As you can see, there are width values > 1696. This does, however,
not mean that you can obtain scanned lines longer than the
width of your scanner or by higher resolutions. Actually, the
resolution is selected by only by the hardware switch. Any line
that is longer than what is defined for the actual resolution
will be undefined (usually white) on the right part that is
exceeding the standard line.
Get current width of the bitmap in pixels.
Set the height of the bitmap in pnm mode. This is actually a
limit on the amount of lines scannable after the first read operation.
When the limit is reached read will return 0. However,
the device is turned off only when a close is performed (either
explicitly or implicitly on exit of the calling process).
Get the current height of the bitmap.
Set the length of the buffer used internally to do the DMA transfer.
The buffer length is supplied in lines of the bitmap.
Since the buffer size limit is (currently) 0x3000 bytes the maximum
number of lines allowed will vary with the width of each
line. This upper limit is checked before it overwrites the current
value and passes an ENOMEM in the errno(2) variable. However,
since the bitmap width can change after a buffer length was
selected a read request may fail with ENOMEM if the buffer length
turns out too high. It is generally wise to choose long buffers
rather than go save in order to obtain better output.
Get the current buffer length in lines.
Set the timeout for the completion of reading one buffer. Since
a handy scanner is a human/computer interface timeout values are
usually higher than those of a flat scanner. Default is 15 seconds.
After timeout is reached the read operation will fail with
EBUSY. Note that the timeout timer starts anew for each buffer
to be read and thus does not cause you to scan faster for longer
images. BLEN/BTIME is similar as MIN/TIME in termios(4).
Get the current buffer timeout.
All ioctl requests that modify a parameter except GSC_SBTIME do not have
an effect on an ongoing scan process, i.e. after the first read request
that follows open. You must close the device and open it again for the
new selections to take effect. Consequently, the selections are not
reset when you close or open the device.
Similarly, requests that read a value do not report the value that is
used for the ongoing scan process. The values needed during the scan
process are saved when it starts and thus are not accessed by ioctl
The BTIME value does, however, have an immediate effect on the ongoing
scan. Thus the timeout can for example be set to long until the user
starts scanning. It can then be set to a short amount to react (nearly)
immediately when the user stops. Note that the user should be left time
to at least fill one buffer without having to haste.
Note that the pbm versus raw mode selection is done by the minor number
not by ioctl requests. In raw mode the selected height of the bitmap
will have no effect.
/dev/gsc0 device node for raw output, has minor number 0.
/dev/gsc0d device node for raw output emitting debug messages if the
GSCDEBUG option was given at compile time, has minor number
/dev/gsc0p device node for output in pbm file format, has minor number
/dev/gsc0pd device node for pbm and debug mode, has minor number 40.
dd if=/dev/gsc0 of=rawfile bs=(width/8) count=(height)
cat /dev/gsc0p > pbmfile
GSCDEBUG When you define this name as an `option' in the kernel configuration
you can get debug output if you access the
driver with a minor number whose debug bit (i.e. bit 5 out
of 7) is set.
cat(1), dd(1), pbmtopgm(1), pnm(1), sgsc(1), close(2), intro(2),
ioctl(2), open(2), read(2), termios(4), pbm(5)
Gunther Schadow <firstname.lastname@example.org>
Even though the scanner device has a little switch by which you should be
able to select one of the four resolution modes, I could not yet determine
how to read its status. Unless this is not fixed the driver depends
on the value passed by means of ioctl(2) which need not match what is
selected by the hardware.
FreeBSD 5.2.1 January 9, 1995 FreeBSD 5.2.1 [ Back ]