Re: [PATCH] exynos-drm: Fix display manager failing to start without IOMMU problem

[Inki Dae]

Hi Shuah,

2016-10-13 8:11 GMT+09:00 Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>:
> Hi Inki,
>
> On 08/15/2016 10:40 PM, Inki Dae wrote:
>
>>>
>>> okay the very first commit that added IOMMU support
>>> introduced the code that rejects non-contig gem memory
>>> type without IOMMU.
>>>
>>> commit 0519f9a12d0113caab78980c48a7902d2bd40c2c
>>> Author: Inki Dae <inki.dae@xxxxxxxxxxx>
>>> Date:   Sat Oct 20 07:53:42 2012 -0700
>>>
>>>     drm/exynos: add iommu support for exynos drm framework
>>>
>
> I haven't given up on this yet. I am still seeing the following failure:
>
> Additional debug messages I added:
> [   15.287403] exynos_drm_gem_create_ioctl() 1
> [   15.287419] exynos_drm_gem_create() flags 1
>
> [   15.311511] [drm:exynos_drm_framebuffer_init] *ERROR* Non-contiguous GEM memory is not supported.
>
> Additional debug message I added:
> [   15.318981] [drm:exynos_user_fb_create] *ERROR* failed to initialize framebuffer
>
> This is what happens:
>
> 1. exynos_drm_gem_create_ioctl() gets called with EXYNOS_BO_NONCONTIG request
> 2. exynos_drm_gem_create(0 goes ahead and creates the GEM buffers
> 3. exynos_user_fb_create() tries to associate GEM to fb and fails during
>    check_fb_gem_memory_type()
>
> At this point, there is no recovery and lightdm fails
>
> xf86-video-armsoc/src/drmmode_exynos/drmmode_exynos.c assumes contiguous
> allocations are not supported in some exynos drm versions: The following
> commit introduced this change:
>
> https://git.linaro.org/arm/xorg/driver/xf86-video-armsoc.git/commitdiff/3be1f6273441fe95dd442f44064387322e16b7e9
>
> excerpts from the diff:-       if (create_gem->buf_type == ARMSOC_BO_SCANOUT)
> -               create_exynos.flags = EXYNOS_BO_CONTIG;
> -       else
> -               create_exynos.flags = EXYNOS_BO_NONCONTIG;
> +
> +       /* Contiguous allocations are not supported in some exynos drm versions.
> +        * When they are supported all allocations are effectively contiguous
> +        * anyway, so for simplicity we always request non contiguous buffers.
> +        */
> +       create_exynos.flags = EXYNOS_BO_NONCONTIG;
>

Above comment, "Contiguous allocations are not supported in some
exynos drm versions.", seems wrong assumption.
The root cause, contiguous allocation is not supported, would be that
they used Linux kernel which didn't have CMA region enough - as
default 16MB, or didn't declare CMA region enough for the DMA device.
So I think they should not force to flag EXYNOS_BO_NONCONTIG and they
should manage the error case if the allocation failed.

> There might have been logic on exynos_drm that forced Contig when it coudn't
> support NONCONTIG. At least, that is what this comment suggests. This assumption
> doesn't appear to be a good one and not sure if this change was made to fix a bug.
>
> After the IOMMU support, this assumption is no longer true. Hence, with IOMMU
> support, latest kernels have a mismatch with the installed xf86-video-armsoc
>
> This is what I am running into. This leads to the following question:
>
> 1. How do we ensure exynos_drm kernel changes don't break user-space
>    specifically xf86-video-armsoc
> 2. This seems to have gone undetected for a while. I see a change in
>    exynos_drm_gem_dumb_create() that is probably addressing this type
>    of breakage. Commit 122beea84bb90236b1ae545f08267af58591c21b adds
>    handling for IOMMU NONCONTIG case.

Seems this patch has a problem. This patch forces to flag NONCONTIG if
iommu is enabled. The flag should be depend on the argument from
user-space.
I.e., if user-space requested a gem allocation with CONTIG flag, then
Exynos drm driver should allocate contiguous memory even though iommu
is enabled.

>
> Anyway, I am interested in getting the exynos_drm kernel side code
> and xf86-video-armsoc in sync to resolve the issue.
>
> Could you recommend a going forward plan?

My opinion are,

1. Do not force to flag EXYNOS_BO_NONCONTIG at xf86-video-armsoc
2. Do not force to flag NONCONTIG at Exynos drm driver even though
iommu is enabled and flag allocation type with the argument from
user-space.

I think you could try to post above patches.

Thanks,
Inki Dae

>
> I can submit a patch to xf86-video-armsoc. I am also looking ahead to
> see if we can avoid such breaks in the future by keeping kernel and
> xf86-video-armsoc in sync.
>
> thanks,
> -- Shuah
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