linux-kernel - Re: [PATCH v1 0/4] msm/gpu/a6xx: use the DMA-API for GMU memory allocations

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Date:   Wed, 19 Feb 2020 21:48:34 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Jordan Crouse <jcrouse@...eaurora.org>
Cc:     linux-arm-msm@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sean Paul <sean@...rly.run>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Stephen Boyd <swboyd@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sharat Masetty <smasetty@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>,
        Rob Clark <robdclark@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Andy Gross <agross@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        freedreno@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v1 0/4] msm/gpu/a6xx: use the DMA-API for GMU memory allocations

On Wed, Feb 19, 2020 at 1:33 PM Jordan Crouse <jcrouse@...eaurora.org> wrote:
>
> When CONFIG_INIT_ON_ALLOC_DEFAULT_ON the GMU memory allocator runs afoul of
> cache coherency issues because it is mapped as write-combine without clearing
> the cache after it was zeroed.
>
> Rather than duplicate the hacky workaround we use in the GEM allocator for the
> same reason it turns out that we don't need to have a bespoke memory allocator
> for the GMU anyway. It uses a flat, global address space and there are only
> two relatively minor allocations anyway. In short, this is essentially what the
> DMA API was created for so replace a bunch of memory management code with two
> calls to allocate and free DMA memory and we're fine.
>
> The only wrinkle is that the memory allocations need to be in a very specific
> location in the GMU virtual address space so in order to get the iova allocator
> to do the right thing we need to specify the dma-ranges property in the device
> tree for the GMU node. Since we've not yet converted the GMU bindings over to
> YAML two patches quickly turn into four but at the end of it we have at least
> one bindings file converted to YAML and 99 less lines of code to worry about.
>
> Jordan Crouse (4):
>   dt-bindings: display: msm: Convert GMU bindings to YAML
>   dt-bindings: display: msm: Add required dma-range property
>   arm64: dts: sdm845: Set the virtual address range for GMU allocations
>   drm/msm/a6xx: Use the DMA API for GMU memory objects

Awesome! Thanks so much for the quick turnaround on this! This set
resolves the crashes I was seeing with
CONFIG_INIT_ON_ALLOC_DEFAULT_ON.

Tested-by: John Stultz <john.stultz@...aro.org>

thanks again!
-john