linux-kernel - Re: [Nouveau] [PATCH v4 1/3] drm/nouveau: set streaming DMA mask early

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Date:   Mon, 3 Oct 2016 14:39:53 +0900
From:   Alexandre Courbot <gnurou@...il.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>
Subject: Re: [Nouveau] [PATCH v4 1/3] drm/nouveau: set streaming DMA mask early

On Mon, Sep 26, 2016 at 9:32 PM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> Some subdevices (i.e., fb/nv50.c and fb/gf100.c) map a scratch page using
> dma_map_page() way before the TTM layer has had a chance to set the DMA
> mask. This may prevent the driver from loading at all on platforms whose
> system memory is not covered by the default DMA mask of 32-bit (i.e., when
> all RAM is above 4 GB).
>
> So set a preliminary DMA mask right after constructing the PCI device, and
> base it on the .dma_bits member of the MMU subdevice, which is what the TTM
> layer will base the DMA mask on as well.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  drivers/gpu/drm/nouveau/nouveau_drm.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
> index 652ab111dd74..e61e9a0adb51 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_drm.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
> @@ -361,6 +361,17 @@ static int nouveau_drm_probe(struct pci_dev *pdev,
>
>         pci_set_master(pdev);
>
> +       /*
> +        * Set a preliminary DMA mask based on the .dma_bits member of the
> +        * MMU subdevice. This allows other subdevices to create DMA mappings
> +        * in their init() function, which are called before the TTM layer sets
> +        * the DMA mask definitively.
> +        * This is necessary for platforms where the default DMA mask of 32
> +        * does not cover any system memory, i.e., when all RAM is > 4 GB.
> +        */
> +       dma_set_mask_and_coherent(device->dev,
> +                                 DMA_BIT_MASK(device->mmu->dma_bits));

I would just move this to nvkm_device_pci_new() so that it perfectly
mirrors the same call done in nvkm_device_tegra_new(), which was done
for the same purpose. Otherwise, looks good to me.