  We simulate a dense athermal suspension of soft particles sheared between
hard walls of a prescribed roughness profile, using a method that fully
accounts for the fluid mechanics of the solvent between the particles, and
between the particles and the walls, as well as for the solid mechanics of
changes in the particle shapes. We thus capture the widely observed phenomenon
of elastohydrodynamic wall slip, in which the soft particles become deformed in
shear and lift away from the wall slightly, leaving behind a thin lubricating
solvent layer of high shear. For imposed stresses below the material's bulk
yield stress, we show the observed wall slip to be dominated by this thin
solvent layer. At higher stresses, it is augmented by an additional
contribution arising from a fluidisation of the first few layers of particles
near the wall. By systematically varying the roughness of the walls, we
quantify a suppression of slip with increasing wall roughness. For smooth
walls, slip radically changes the steady state bulk flow curve of shear stress
as a function of shear rate, by conferring a branch of apparent (slip-induced)
flow even for $\sigma<\sigma_y$, as seen experimentally. We also elucidate the
effects of slip on the dynamics of yielding following the imposition of a
constant shear stress, characterising the timescales at which bulk yielding
arises, and at which slip first sets in.
