Modem CMUX: Optimize away work buf #93454
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The cmux work buffer, originally intended as a simple optimization for receiving data in small chunks rather than 1 byte at a time from through the modem_pipe_receive() API, is now being used as a global baseline for the sizes of receive buffers. This is an issue given this means the work buffer of the CMUX instance has grown be large enough to store a whole CMUX frame, which is not neccesary. An optimization to the modem_cmux module makes the work buffer itself obsolete, as the data receive buffer (the receive_buf passed to the modem_cmux instance) can be used as the temporary buffer to store chunks. We simply receive the data from the pipe, to the remaining part of the receive buffer, which will always line up with the current frame being received. To do this trick, all we need to ensure is that the receive buffer is large enough to store at least MTU + the CMUX header and footer, which is already the case. To make this super clear, the struct modem_cmux_config has been expanded to mention these restrictions and why they are there. The already present __ASSERTs will make sure the receive buffer is sized appropriately. With this change, the previously mentioned parts using the work buffer size to determine receive buffer sizes, have been updated to follow the MTU + constant described in the struct modem_cmux_config. Signed-off-by: Bjarki Arge Andreasen <[email protected]>
Add entry in migration guide for 4.3 which mentions why the config was removed and what to replace uses of the config with. Signed-off-by: Bjarki Arge Andreasen <[email protected]>
The MTU used by CMUX was set by the config CONFIG_MODEM_CMUX_MTU. This means that all CMUX instances must use the same MTU size. Different CMUX instances need to be configured with an MTU that matches the CMUX instance they are connected to. Additionally, mechanisms outside of CMUX may be used to negotiate an MTU size at runtime, this is not possible if the MTU size is specified fixed at build time. This commit refactors modem_cmux and the modem_cellular driver to pass the MTU when the modem_cmux instance is initialized. Signed-off-by: Bjarki Arge Andreasen <[email protected]>
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Work towards removing global buffer sizes for modem modules.
Related PRs: #93441