Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20181129140315.28476-1-vivek.gautam@codeaurora.org> <20181129141429.GA22638@lst.de>
In-Reply-To: <20181129141429.GA22638@lst.de>
From:   Rob Clark <robdclark@gmail.com>
Date:   Thu, 29 Nov 2018 09:25:43 -0500
Message-ID: <CAF6AEGvsFbXGkC+=4oq1xXnoC2EVX2J0k-GqShefcek1=uw4Lw@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*
To:     hch@lst.de
Cc:     Vivek Gautam <vivek.gautam@codeaurora.org>,
        David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Tomasz Figa <tfiga@chromium.org>,
        Archit Taneja <architt@codeaurora.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Jordan Crouse <jcrouse@codeaurora.org>,
        Sean Paul <seanpaul@chromium.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Nov 29, 2018 at 9:14 AM Christoph Hellwig <hch@lst.de> wrote:
>
> On Thu, Nov 29, 2018 at 07:33:15PM +0530, Vivek Gautam wrote:
> > dma_map_sg() expects a DMA domain. However, the drm devices
> > have been traditionally using unmanaged iommu domain which
> > is non-dma type. Using dma mapping APIs with that domain is bad.
> >
> > Replace dma_map_sg() calls with dma_sync_sg_for_device{|cpu}()
> > to do the cache maintenance.
>
> As I told you before:  hell no.  If you spent the slightest amount of
> actually trying to understand what you are doing here you'd know this
> can't work.  Just turn on dma debugging and this will blow up in your
> face.

you can tone it down..  we weren't the ones who created the dma/iommu
mess, we are just trying to find a way to work around it

> Either you use the DMA API properly, that is you use it to map and
> to sync, or you don't use it at all.  Mix and match between iommu
> APIs and DMA APIs is simply not possible.

I'd *love* nothing more to not use the dma api.. but on arm there is
no other way to do cache maint.

BR,
-R