  We report the results of AstroSat observations of GRS 1915$+$105 obtained
using 100 ks guaranteed-time (GT) during the soft state. The Color-Color
Diagram (CCD) indicates a variability class of $\delta$ with the detection of
High Frequency QPO (HFQPO) in the power density spectra (PDS). The HFQPO is
seen to vary in the frequency range of $67.96 - 70.62$ Hz with percentage rms
$\sim 0.83 - 1.90$ % and significance varying from $1.63 - 7.75$. The energy
dependent power spectra show that the HFQPO features are dominant only in $6 -
25$ keV energy band. The broadband energy spectra ($0.7 - 50$ keV) of SXT (Soft
X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) modelled
with nthComp and powerlaw imply that the source has an extended corona in
addition to a compact 'Comptonizing corona' that produces high energy emission
and exhibits HFQPOs. The broadband spectral modelling indicates that the source
spectra are well described by thermal Comptonization with electron temperature
(kT$_{\rm e}$) of $2.07 - 2.43$ keV and photon-index ($\Gamma_{\rm nth}$)
between $1.73-2.45$ with an additional powerlaw component of photon-index
($\Gamma_{\rm PL}$) between $2.94 - 3.28$. The norm of nthComp component is
high ($\sim 8$) during the presence of strong HFQPO and low ($\sim 3$) during
the absence of HFQPO. Further, we model the energy spectra with the kerrbb
model to estimate the accretion rate, mass and spin of the source. Our findings
indicate that the source accretes at super-Eddington rate of $1.17-1.31~
\dot{M}_{\rm Edd}$. Moreover, we find the mass and spin of the source as $12.44
- 13.09~M_{\odot}$ and $0.990-0.997$ with $90\%$ confidence suggesting that GRS
1915$+$105 is a maximally rotating stellar mass X-ray binary black hole source.
