  We use a range of cosmological data to constrain phenomenological
modifications to general relativity on cosmological scales, through
modifications to the Poisson and lensing equations. We include cosmic microwave
background anisotropy measurements from the Planck satellite, cosmic shear from
CFHTLenS and DES-SV, and redshift-space distortions from BOSS data release 12
and the 6dF galaxy survey. We find no evidence of departures from general
relativity, with the modified gravity parameters constrained to $\Sigma_0 =
0.05^{+0.05}_{-0.07}$ and $\mu_0 = -0.10^{+0.20}_{-0.16}$, where $\Sigma_0$ and
$\mu_0$ refer to deviations from general relativity today and are defined to be
zero in general relativity. We also forecast the sensitivity to those
parameters of the full five-year Dark Energy Survey and of an experiment like
the Large Synoptic Survey Telescope, showing a substantial expected improvement
in the constraint on $\Sigma_0$.
