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