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

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Message-ID: <20190121150734.GA30582@infradead.org>
Date:   Mon, 21 Jan 2019 07:07:34 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, 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

> +#include <linux/dma-noncoherent.h>

This header is not for usage in device drivers, but purely for
dma-mapping implementations!

> +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.