DSD Recording on RME 2-PRO FS/BE

Greetings,

I am testing native DSD recording using arecord with the RME 2-PRO FS/BE audio interface.

using command line

arecord -c 2 -Dhw:CARD=Pro53791340 -r 176400 -f DSD_U32_BE audio.dsf

gives output of:

Recording WAVE 'audio.dsf' : Direct Stream Digital, 4-byte (x32), big endian, oldest bits in MSB, Rate 176400 Hz, Stereo arecord: set_params:1368: Sample format non available Available formats: - S32_LE

Using S32_LE parameter arecord -c 2 -Dhw:CARD=Pro53791340 -r 176400 -f S32_LE audio.dsf

gives output of

Recording WAVE 'audio.dsf' : Signed 32 bit Little Endian, Rate 176400 Hz, Stereo

Which appears to be a PCM recording and not DSD. What is the correct command line to arecord to capture DSD audio streams and write them to a dsd file?

Thanks

David