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    Subject[PATCH 29/30] 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 <>
    Signed-off-by: Evgeniy Polyakov <>
    Cc: Randy Dunlap <>
    Signed-off-by: Andrew Morton <>
    Documentation/w1/masters/ds2490 | 52 +++++++++++++++++++++++++++++++++++++++
    1 files changed, 52 insertions(+), 0 deletions(-)

    diff --git a/Documentation/w1/masters/ds2490 b/Documentation/w1/masters/ds2490
    index 239f9ae..28176de 100644
    --- a/Documentation/w1/masters/ds2490
    +++ b/Documentation/w1/masters/ds2490
    @@ -16,3 +16,55 @@ 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 <>

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