| commit | 02b2c0a6067cc654457be84ec5faa6ea8e8bdcbc | [log] [tgz] |
|---|---|---|
| author | Jes B. Klinke <jbk@chromium.org> | Tue May 24 14:21:19 2022 -0700 |
| committer | cfrantz <frantzcj+github@gmail.com> | Thu Jun 09 12:03:57 2022 -0700 |
| tree | 9313c386d723845cb1e44a7bb1873fcf2b294faf | |
| parent | 97ff9d74d47328fc9a0a17947ec1a084ed437776 [diff] |
[opentitantool]: Allow SPI chip select across transactions
While implementing TPM communication, I came across a use case in which
I wanted to perform a few SPI transfers, inspect the data that was read,
and then possibly issue some more SPI transfers, while keeping CS
asserted throughout.
The current `spi::Target` interface is such that `run_transaction()`
keeps CS asserted for the duration of the list of transfers given in the
parameter list and then deasserts it unconditionally, with no option to
add more based on the result of the previous ones.
This PR introduces an explicit `assert_cs()` function, which can be used
to keep CS asserted across multiple invocations of `run_transaction()`,
like below.
```
let spi = ...;
{
let _cs_asserted = spi.assert_cs()?;
spi.run_transaction(...)?;
if ... {
spi.run_transaction(...)?;
}
// CS will automatically deassert here (or earlier if bailing out.)
}
```
Furthermore, `assert_cs()` counts its invocations, and releases CS only
when every nested assertion has been released, such that if the above
code was part of a helper method, and the called wanted to make several
invocations of the helper, during a single CS session, then it could put
another `assert_cs()` around its logic.
Signed-off-by: Jes B. Klinke <jbk@chromium.org>
Change-Id: I219f3647d29129c4dfc1fc346f855fb4e9da2e53
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