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-Kernel driver ds2490
- * Maxim DS2490 based
-Author: Evgeniy Polyakov <email@example.com>
-The Maxim/Dallas Semiconductor DS2490 is a chip
-which allows to build USB <-> W1 bridges.
-DS9490(R) is a USB <-> W1 bus master device
-which has 0x81 family ID integrated chip and DS2490
-low-level operational chip.
-Notes and limitations.
-- The weak pullup current is a minimum of 0.9mA and maximum of 6.0mA.
-- The 5V strong pullup is supported with a minimum of 5.9mA and a
- maximum of 30.4 mA. (From DS2490.pdf)
-- While the ds2490 supports a hardware search the code doesn't take
- advantage of it (in tested case it only returned first device).
-- The hardware will detect when devices are attached to the bus on the
- next bus (reset?) operation, however only a message is printed as
- the core w1 code doesn't make use of the information. Connecting
- one device tends to give multiple new device notifications.
-- The number of USB bus transactions could be reduced if w1_reset_send
- was added to the API. The name is just a suggestion. It would take
- a write buffer and a read buffer (along with sizes) as arguments.
- The ds2490 block I/O command supports reset, write buffer, read
- buffer, and strong pullup all in one command, instead of the current
- 1 reset bus, 2 write the match rom command and slave rom id, 3 block
- write and read data. The write buffer needs to have the match rom
- command and slave rom id prepended to the front of the requested
- write buffer, both of which are known to the driver.
-- The hardware supports normal, flexible, and overdrive bus
- communication speeds, but only the normal is supported.
-- The registered w1_bus_master functions don't define error
- conditions. If a bus search is in progress and the ds2490 is
- removed it can produce a good amount of error output before the bus
- search finishes.
-- The hardware supports detecting some error conditions, such as
- short, alarming presence on reset, and no presence on reset, but the
- driver doesn't query those values.
-- The ds2490 specification doesn't cover short bulk in reads in
- detail, but my observation is if fewer bytes are requested than are
- available, the bulk read will return an error and the hardware will
- clear the entire bulk in buffer. It would be possible to read the
- maximum buffer size to not run into this error condition, only extra
- bytes in the buffer is a logic error in the driver. The code should
- should match reads and writes as well as data sizes. Reads and
- writes are serialized and the status verifies that the chip is idle
- (and data is available) before the read is executed, so it should
- not happen.
-- Running x86_64 2.6.24 UHCI under qemu 0.9.0 under x86_64 2.6.22-rc6
- with a OHCI controller, ds2490 running in the guest would operate
- normally the first time the module was loaded after qemu attached
- the ds2490 hardware, but if the module was unloaded, then reloaded
- most of the time one of the bulk out or in, and usually the bulk in
- would fail. qemu sets a 50ms timeout and the bulk in would timeout
- even when the status shows data available. A bulk out write would
- show a successful completion, but the ds2490 status register would
- show 0 bytes written. Detaching qemu from the ds2490 hardware and
- reattaching would clear the problem. usbmon output in the guest and
- host did not explain the problem. My guess is a bug in either qemu
- or the host OS and more likely the host OS.
--- 03-06-2008 David Fries <David@Fries.net>