modem: cmux: Decouple modem cmux and modem cellular driver #93441
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| help | ||
| Use the default MTU size of 127 bytes for the CMUX module on certain modems. | ||
| This must match the AT+CMUX commands in the modem_cellular driver. | ||
| Hint that the default MODEM_CMUX_MTU size should be 127 bytes. |
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No it's not just a hint? It's enforced by the CMUX module?
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its enforced by MODEM_CMUX_MTU :) The default is just a default, so this hint is telling CMUX that something in the system would prefer MTU be 127, but it is definitely still directly settable by the application
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Looking a bit closer, what does MODEM_CMUX_MTU do? It seems like an artificial limit, given the actual limit of CMUX frame sizes is MODEM_CMUX_WORK_BUFFER_SIZE, where is it used favorably to MODEM_CMUX_WORK_BUFFER_SIZE?
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I see, we have no way to configure the MTU used by CMUX, major oversight on my end when creating the API...
The struct modem_cmux_config should be updated to include the MTU, this way, it becomes instance specific and needs no global config. On top of that, part of the receive buffer should be reserved for the work buffer. These two changes remove all hardcoded configs from the CMUX implementation :)
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Seems good. FYI @SeppoTakalo @MarkusLassila
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CMUX frame sizes were last worked on PR #87115
And it is configurable by Kconfig.
Surely it can be a runtime configurable, but does it really make much difference? You have selected the modem already in the build time.
Some modems use hard-coded MTU for the CMUX. Some allow it to be configured in the AT command that enables the CMUX framing.
Even some clients hard-code the value. For example Linux ldattach uses 127.
3GPP default is 32.
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Looking a bit closer, what does
MODEM_CMUX_MTUdo? It seems like an artificial limit
It is not an artificial limit, frames bigger than the MTU what the receiver can process, are dropped.
It is not directly related to how much we have buffer space. It depends on the remote end as well.
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The point is not to make the MTU runtime configurable, but to make it instance specific instead a global property.
The modem modules cmux module is currently directly coupled to the presence of specific modems, rather than being selected by drivers for whatever hardware wants to request the default MTU of 127 bytes. This commit the makes the device drivers (for now, modem_cellular) select the symbol, thus decoupling the modem modules from the presence of any specific device. Signed-off-by: Bjarki Arge Andreasen <[email protected]>
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Still typo? 😄 |
The modem modules cmux module is currently directly coupled to the presence of specific modems, rather than being selected by drivers for whatever hardware wants to request the default MTU of 127 bytes.
This commit moves the makes the device drivers (for now, modem_cellular) select the symbol, thus decoupling the modem modules from the presence of any specific device.