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| -rw-r--r-- | 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 239f9ae01843..28176def3d6f 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 <David@Fries.net> |