Here is my patch proposal.
I have a warning using checkpatch.pl:
WARNING: static const char * array should probably be static const char * const
#21: FILE: drivers/atmodem/gprs-context.c:50:
+static const char *cgdata_prefix[] = { "+CGDATA:", NULL };

I have copied what is in src/gprs.c. Let me know if it should be fixed.

Christophe Ronco (1):
  atmodem: use ATD99 to enter data state when needed

 drivers/atmodem/gprs-context.c | 45 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

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2.11.0