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

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Date:   Mon, 21 Jan 2019 16:33:27 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org,
        Christian Koenig <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        David Zhou <David1.Zhou@....com>,
        Huang Rui <ray.huang@....com>,
        Junwei Zhang <Jerry.Zhang@....com>,
        Michel Daenzer <michel.daenzer@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent
 devices on non-x86

On Mon, 21 Jan 2019 at 16:07, Christoph Hellwig <hch@...radead.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?