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

From: Alexandre Courbot
Date: Sat Jul 16 2016 - 02:20:34 EST

On Sat, Jul 16, 2016 at 4:45 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 15 July 2016 at 07:52, Alexandre Courbot <gnurou@xxxxxxxxx> wrote:
>> On Fri, Jul 8, 2016 at 1:59 AM, Ard Biesheuvel
>> <ard.biesheuvel@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> ---
>>> 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?
> Yes. This particular device supports 40 bits (judging from the MMU
> driver code) of physical address space, and RAM starts at
> 0x80_0000_0000 on AMD Seattle, so we need all 40 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).
> Happy to have stab at implementing this, but I'd like some buy in from
> the maintainer first before I dive into this. Ben is the person to
> give his blessing, I suppose? He has not responded to any of my
> postings so far, unfortunately.

A patch would make it easier to judge whether this is the right thing
to do, but let's hear what Ben thinks about it.