  The linear-$T$ resistivity is one of the hallmarks of various strange metals
regardless of their microscopic details. Towards understanding this universal
property, the holographic method or gauge/gravity duality has made much
progress. Most holographic models have focused on the low temperature limit,
where the linear-$T$ resistivity has been explained by the infrared geometry.
We extend this analysis to high temperature and identify the conditions for a
robust linear-$T$ resistivity up to high temperature. This extension is
important because, in experiment, the linear-$T$ resistivity is observed in a
large range of temperatures, up to room temperature. In the axion-dilaton
theories we find that, to have a robust linear-$T$ resistivity, the strong
momentum relaxation is a necessary condition, which agrees with the previous
result for the Guber-Rocha model. However, it is not sufficient in the sense
that, among large range of parameters giving a linear-$T$ resistivity in low
temperature limit, only very limited parameters can support the linear-$T$
resistivity up to high temperature even in strong momentum relaxation. We also
show that the incoherent term in the general holographic conductivity formula
or the coupling between the dilaton and Maxwell term is responsible for a
robust linear-$T$ resistivity up to high temperature.
