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authorDavid Fries <david@fries.net>2008-10-16 01:05:09 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2008-10-16 14:21:51 -0400
commit3823ee44cfa8b0e6edbc0c21b81b49b95a27ca0d (patch)
tree19bfac49e8e681666ff8b3057f8fe9a130e87ed3 /Documentation
parentcbf4a49afa9f9a527ed4f3dab4ec355586ba890e (diff)
W1: Documentation/w1/masters/ds2490 update
Provide some additional details about the status of the driver and the ds2490 hardware. Signed-off-by: David Fries <david@fries.net> Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru> Cc: Randy Dunlap <randy.dunlap@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/w1/masters/ds249052
1 files changed, 52 insertions, 0 deletions
diff --git a/Documentation/w1/masters/ds2490 b/Documentation/w1/masters/ds2490
index 239f9ae01843..28176def3d6f 100644
--- a/Documentation/w1/masters/ds2490
+++ b/Documentation/w1/masters/ds2490
@@ -16,3 +16,55 @@ which allows to build USB <-> W1 bridges.
16DS9490(R) is a USB <-> W1 bus master device 16DS9490(R) is a USB <-> W1 bus master device
17which has 0x81 family ID integrated chip and DS2490 17which has 0x81 family ID integrated chip and DS2490
18low-level operational chip. 18low-level operational chip.
19
20Notes and limitations.
21- The weak pullup current is a minimum of 0.9mA and maximum of 6.0mA.
22- The 5V strong pullup is supported with a minimum of 5.9mA and a
23 maximum of 30.4 mA. (From DS2490.pdf)
24- While the ds2490 supports a hardware search the code doesn't take
25 advantage of it (in tested case it only returned first device).
26- The hardware will detect when devices are attached to the bus on the
27 next bus (reset?) operation, however only a message is printed as
28 the core w1 code doesn't make use of the information. Connecting
29 one device tends to give multiple new device notifications.
30- The number of USB bus transactions could be reduced if w1_reset_send
31 was added to the API. The name is just a suggestion. It would take
32 a write buffer and a read buffer (along with sizes) as arguments.
33 The ds2490 block I/O command supports reset, write buffer, read
34 buffer, and strong pullup all in one command, instead of the current
35 1 reset bus, 2 write the match rom command and slave rom id, 3 block
36 write and read data. The write buffer needs to have the match rom
37 command and slave rom id prepended to the front of the requested
38 write buffer, both of which are known to the driver.
39- The hardware supports normal, flexible, and overdrive bus
40 communication speeds, but only the normal is supported.
41- The registered w1_bus_master functions don't define error
42 conditions. If a bus search is in progress and the ds2490 is
43 removed it can produce a good amount of error output before the bus
44 search finishes.
45- The hardware supports detecting some error conditions, such as
46 short, alarming presence on reset, and no presence on reset, but the
47 driver doesn't query those values.
48- The ds2490 specification doesn't cover short bulk in reads in
49 detail, but my observation is if fewer bytes are requested than are
50 available, the bulk read will return an error and the hardware will
51 clear the entire bulk in buffer. It would be possible to read the
52 maximum buffer size to not run into this error condition, only extra
53 bytes in the buffer is a logic error in the driver. The code should
54 should match reads and writes as well as data sizes. Reads and
55 writes are serialized and the status verifies that the chip is idle
56 (and data is available) before the read is executed, so it should
57 not happen.
58- Running x86_64 2.6.24 UHCI under qemu 0.9.0 under x86_64 2.6.22-rc6
59 with a OHCI controller, ds2490 running in the guest would operate
60 normally the first time the module was loaded after qemu attached
61 the ds2490 hardware, but if the module was unloaded, then reloaded
62 most of the time one of the bulk out or in, and usually the bulk in
63 would fail. qemu sets a 50ms timeout and the bulk in would timeout
64 even when the status shows data available. A bulk out write would
65 show a successful completion, but the ds2490 status register would
66 show 0 bytes written. Detaching qemu from the ds2490 hardware and
67 reattaching would clear the problem. usbmon output in the guest and
68 host did not explain the problem. My guess is a bug in either qemu
69 or the host OS and more likely the host OS.
70-- 03-06-2008 David Fries <David@Fries.net>