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

[Randy Dunlap]

On Fri, 11 Jul 2008 17:51:44 +0400 Evgeniy Polyakov wrote:

> Provide some additional details about the status of the driver and the
> ds2490 hardware.
> 
> Signed-off-by: David Fries <david@xxxxxxxxx>
> Signed-off-by: Evgeniy Polyakov <johnpol@xxxxxxxxxxx>
> 
> ---
>  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,
                  ~~~~~~~~~~~~~~~~~
Can you clarify above?

> +  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,

                                                                   are:
                                            or (no comma)          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
                           ~~~~~~~~~~~~~
                           (duplicated)

> +  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

                                   flexible,

> +  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@xxxxxxxxx>
> -- 


---
~Randy
Linux Plumbers Conference, 17-19 September 2008, Portland, Oregon USA
http://linuxplumbersconf.org/
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