linux-kernel - Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

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Message-ID: <20181220153719.GB22997@infradead.org>
Date:   Thu, 20 Dec 2018 07:37:19 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Oleksandr Andrushchenko <andr2000@...il.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, daniel.vetter@...el.com,
        jgross@...e.com, boris.ostrovsky@...cle.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

On Wed, Dec 19, 2018 at 02:14:52PM +0100, Gerd Hoffmann wrote:
> 
> > > > +    if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > > > +            DMA_BIDIRECTIONAL)) {
> > > 
> > > 
> > > Are you using the DMA streaming API as a way to flush the caches?
> > Yes
> > > Does this mean that GFP_USER isn't making the buffer coherent?
> > 
> > No, it didn't help. I had a question [1] if there are any other better way
> > to achieve the same, but didn't have any response yet. So, I implemented
> > it via DMA API which helped.
> 
> set_pages_array_*() ?
> 
> See arch/x86/include/asm/set_memory.h

That sounds even more bogus, don't go there.