[RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86

[Ard Biesheuvel]

On Mon, 21 Jan 2019 at 16:07, Christoph Hellwig <hch at infradead.org> wrote:
>
> > +#include <linux/dma-noncoherent.h>
>
> This header is not for usage in device drivers, but purely for
> dma-mapping implementations!
>

Is that documented anywhere?

> > +static inline bool drm_device_can_wc_memory(struct drm_device *ddev)
> >  {
> > +     if (IS_ENABLED(CONFIG_PPC))
> > +             return IS_ENABLED(CONFIG_NOT_COHERENT_CACHE);
> > +     else if (IS_ENABLED(CONFIG_MIPS))
> > +             return !IS_ENABLED(CONFIG_CPU_LOONGSON3);
> > +     else if (IS_ENABLED(CONFIG_X86))
> > +             return true;
> > +
> > +     return !dev_is_dma_coherent(ddev->dev);
>
> And even if something like this was valid to do, it would have to be
> a core function with an arch hook, and not hidden in a random driver.

Why would it not be valid to do? Is it wrong for a driver to be aware
of whether a device is cache coherent or not?

And in case it isn't, do you have an alternative suggestion on how to
fix this mess?