linux-kernel - Re: [Nouveau] [PATCH v3] drm/nouveau/fb/nv50: set DMA mask before mapping scratch page

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Message-ID: <CAAVeFuL+NEigyYk2LK3O5PLmeOSEOAKBpoT7TDbsvArWhtaA=g@mail.gmail.com>
Date:	Fri, 15 Jul 2016 14:52:22 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	"nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	David Airlie <airlied@...ux.ie>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Nouveau] [PATCH v3] drm/nouveau/fb/nv50: set DMA mask before
 mapping scratch page

On Fri, Jul 8, 2016 at 1:59 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> The 100c08 scratch page is mapped using dma_map_page() before the TTM
> layer has had a chance to set the DMA mask. This means we are still
> running with the default of 32 when this code executes, and this causes
> problems for platforms with no memory below 4 GB (such as AMD Seattle)
>
> So move the dma_map_page() to the .init hook, and set the streaming DMA
> mask based on the MMU subdev parameters before performing the call.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
> I am sure there is a much better way to address this, but this fixes the
> problem I get on AMD Seattle with a GeForce 210 PCIe card:
>
>    nouveau 0000:02:00.0: enabling device (0000 -> 0003)
>    nouveau 0000:02:00.0: NVIDIA GT218 (0a8280b1)
>    nouveau 0000:02:00.0: bios: version 70.18.a6.00.00
>    nouveau 0000:02:00.0: fb ctor failed, -14
>    nouveau: probe of 0000:02:00.0 failed with error -14
>
> v2: replace incorrect comparison of dma_addr_t type var against NULL
> v3: rework code to get rid of DMA_ERROR_CODE references, which is not
>     defined on all architectures
>
>  drivers/gpu/drm/nouveau/nvkm/subdev/fb/nv50.c | 40 ++++++++++++++------
>  1 file changed, 29 insertions(+), 11 deletions(-)

I think the same problem exists in fb/gf100.c, would be nice to fix it
there as well.

I have faced similar issues on Tegra before. I wonder whether this
could not be addressed the same way I did, i.e. by setting a
temporary, fail-safe DMA mask in nvkm_device_pci_new()? That would
allow all subdevs to map pages to the device safely in their init.
With your solution, each subdev in that scenario needs to set a DMA
mask to be safe.

Not sure whether that's practical as I suppose you want to make the
DMA mask larger than 32 bits?

If you absolutely need to do this in the device, can we move the DMA
mask setting logic in nouveau_ttm into its own function and call it
from the FB driver to make sure the mask is correctly set? Maybe this
could even be made a MMU function and called during MMU ctor or init
(in the latter case we would also need to reorder MMU init to make it
happen before FB and INSTMEM).