9ca5a1838e
Tokens are sequence numbers embedded in the each SCMI message header: they are used to correlate commands with responses (and delayed responses), but their usage and policy of selection is entirely up to the caller (usually the OSPM agent), while they are completely opaque to the callee (i.e. SCMI platform) which merely copies them back from the command into the response message header. This also means that the platform does not, can not and should not enforce any kind of policy on received messages depending on the contained sequence number: platform can perfectly handle concurrent requests carrying the same identifiying token if that should happen. Moreover the platform is not required to produce in-order responses to agent requests, the only constraint in these regards is that in case of an asynchronous message the delayed response must be sent after the immediate response for the synchronous part of the command transaction. Currenly the SCMI stack of the OSPM agent selects a token for the egressing commands picking the lowest possible number which is not already in use by an existing in-flight transaction, which means, in other words, that we immediately reuse any token after its transaction has completed or it has timed out: this policy indeed does simplify management and lookup of tokens and associated xfers. Under the above assumptions and constraints, since there is really no state shared between the agent and the platform to let the platform know when a token and its associated message has timed out, the current policy of early reuse of tokens can easily lead to the situation in which a spurious or late received response (or delayed_response), related to an old stale and timed out transaction, can be wrongly associated to a newer valid in-flight xfer that just happens to have reused the same token. This misbehaviour on such late/spurious responses is more easily exposed on those transports that naturally have an higher level of parallelism in processing multiple concurrent in-flight messages. This commit introduces a new policy of selection of tokens for the OSPM agent: each new command transfer now gets the next available, monotonically increasing token, until tokens are exhausted and the counter rolls over. Such new policy mitigates the above issues with late/spurious responses since the tokens are now reused as late as possible (when they roll back ideally) and so it is much easier to identify such late/spurious responses to stale timed out transactions: this also helps in simplifying the specific transports implementation since stale transport messages can be easily identified and discarded early on in the rx path without the need to cross check their actual state with the core transport layer. This mitigation is even more effective when, as is usually the case, the maximum number of pending messages is capped by the platform to a much lower number than the whole possible range of tokens values (2^10). This internal policy change in the core SCMI transport layer is fully transparent to the specific transports so it has not and should not have any impact on the transports implementation. Link: https://lore.kernel.org/r/20210803131024.40280-5-cristian.marussi@arm.com Signed-off-by: Cristian Marussi <cristian.marussi@arm.com> Signed-off-by: Sudeep Holla <sudeep.holla@arm.com> |
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| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.