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Date:   Thu, 8 Aug 2019 09:59:47 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Rob Clark <robdclark@chromium.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        Rob Clark <robdclark@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Allison Randal <allison@lohutok.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] drm: add cache support for arm64
Message-ID: <20190808075947.GE30308@lst.de>
References: <20190805211451.20176-1-robdclark@gmail.com> <20190806084821.GA17129@lst.de> <CAJs_Fx6eh1w7c=crMoD5XyEOMzP6orLhqUewErE51cPGYmObBQ@mail.gmail.com> <20190806143457.GF475@lakrids.cambridge.arm.com> <CAJs_Fx4h6SWGmDTLBnV4nmWUFAs_Ge1inxd-dW9aDKgKqmc1eQ@mail.gmail.com> <20190807123807.GD54191@lakrids.cambridge.arm.com> <CAJs_Fx5xU2-dn3iOVqWTzAjpTaQ8BBNP_Gn_iMc-eJpOX+iXoQ@mail.gmail.com> <20190807164958.GA44765@lakrids.cambridge.arm.com> <CAJs_Fx71T=kJEgt28TWqzw+jOahSbLQynCg83+szQW7op4xBkQ@mail.gmail.com>
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On Wed, Aug 07, 2019 at 10:30:04AM -0700, Rob Clark wrote:
> So, we do end up using GFP_HIGHUSER, which appears to get passed thru
> when shmem gets to the point of actually allocating pages.. not sure
> if that just ends up being a hint, or if it guarantees that we don't
> get something in the linear map.
> 
> (Bear with me while I "page" this all back in.. last time I dug thru
> the shmem code was probably pre-armv8, or at least before I had any
> armv8 hw)

GFP_HIGHUSER basically just means that this is an allocation that could
dip into highmem, in which case it would not have a kernel mapping.
This can happen on arm + LPAE, but not on arm64.