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GEHC CS ONE (codename is PPD), has multiple microcontrollers connected
via UART controlling. UART2 is connected to an on-board microcontroller
at 19200 baud, which constantly pushes critical data (so aging character
detect interrupt will never trigger). This data must be processed at
50-200 Hz, so UART should return data in less than 5-20ms. With 1024
byte DMA buffer (and a constant data stream) the read operation instead
needs 1024 byte / 19200 baud = 53.333ms, which is way too long (note:
Worst case would be remote processor sending data with short pauses <=
7 characters, which would further increase this number). The current
downstream kernel instead configures 24 bytes resulting in 1.25ms, but
that is obviously not sensible for normal UART use cases and cannot be
used as new default.
The same device also has another microcontroller with a 4M baud connected
to UART5 exchanging lots of data. For this the same mechanism can be used
to increase the buffer size (downstream uses 4K instead of the default 1K)
with potentially slightly reduced buffer count. At this baud rate latency
is not an issue (4096 byte / 4M baud = 0.977 ms). Before increasing the
default buffer count from 4 to 16 in
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