[PATCH 29/30] W1: Documentation/w1/masters/ds2490 update

Evgeniy Polyakov
2008-07-11T13:56:59Z

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>

---
Documentation/w1/masters/ds2490 | 62 +++++++++++++++++++++++++++++++++++++++
1 files changed, 62 insertions(+), 0 deletions(-)

diff --git a/Documentation/w1/masters/ds2490 b/Documentation/w1/masters/ds2490
index 239f9ae..f70b8dc 100644
--- a/Documentation/w1/masters/ds2490
+++ b/Documentation/w1/masters/ds2490
@@ -16,3 +16,65 @@ 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.
+ It would add match rom and rom to the send buffer, reset the bus,
+ and read the result. The ds2490 block I/O command supports reset,
+ write, read, and strong pullup all in one command. That would
+ reduce the time and overhead for a set of commands.
+
+ The core w1 functions required to do a temperature conversion are,
+ w1_reset_select_slave, w1_next_pullup, and w1_write_8, which turn
+ around and execute the master functions, reset_bus, write_block,
+ set_pullup, write_byte, set_pullup. Each ds2490 function will have
+ multiple USB bus transactions, except set_pullup where it only
+ requires transactions if the if the time value is different. The
+ conversion could be reduced to w1_next_pullup, w1_reset_send which
+ would call set_pullup, reset_send. The reset_send would enable reset
+ and strong pullup (if enabled), write the data buffer, execute the
+ block I/O command, read status, and get data. Currently I count 11
+ transactions required, this would reduce it to 4.
+- The hardware supports normal, flexable, 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>
--
1.4.4.4