[PATCH] drm/msm: Check for the GPU IOMMU during bind

[Dmitry Baryshkov]

On 20/07/2023 18:52, Rob Clark wrote:
> On Thu, Jul 6, 2023 at 11:55 AM Dmitry Baryshkov
> <dmitry.baryshkov at linaro.org> wrote:
>>
>> On 10/03/2023 00:20, Jordan Crouse wrote:
>>> While booting with amd,imageon on a headless target the GPU probe was
>>> failing with -ENOSPC in get_pages() from msm_gem.c.
>>>
>>> Investigation showed that the driver was using the default 16MB VRAM
>>> carveout because msm_use_mmu() was returning false since headless devices
>>> use a dummy parent device. Avoid this by extending the existing is_a2xx
>>> priv member to check the GPU IOMMU state on all platforms and use that
>>> check in msm_use_mmu().
>>>
>>> This works for memory allocations but it doesn't prevent the VRAM carveout
>>> from being created because that happens before we have a chance to check
>>> the GPU IOMMU state in adreno_bind.
>>>
>>> There are a number of possible options to resolve this but none of them are
>>> very clean. The easiest way is to likely specify vram=0 as module parameter
>>> on headless devices so that the memory doesn't get wasted.
>>
>> This patch was on my plate for quite a while, please excuse me for
>> taking it so long.
>>
>> I see the following problem with the current code. We have two different
>> instances than can access memory: MDP/DPU and GPU. And each of them can
>> either have or miss the MMU.
>>
>> For some time I toyed with the idea of determining whether the allocated
>> BO is going to be used by display or by GPU, but then I abandoned it. We
>> can have display BOs being filled by GPU, so handling it this way would
>> complicate things a lot.
> 
> There is MSM_BO_SCANOUT .. but it wouldn't completely surprise me if
> it isn't used in some place where it should somewhere or other.  But
> that is the hint that contiguous allocation should be used if the
> display doesn't support some sort of iommu.  (Using a GPU without some
> sort of mmu/iommu isn't something sane to do.. the only reason the
> support for that exists at all is to aid bringup.  I wouldn't call
> that a "supported" configuration.)
> 
>> This actually rings a tiny bell in my head with the idea of splitting
>> the display and GPU parts to two different drivers, but I'm not sure
>> what would be the overall impact.
> 
> Userspace does have better support for split display/gpu these days
> than it did when drm/msm was first merged.  It _might_ just work if
> one device only advertised DRIVER_RENDER and the other
> MODESET/ATOMIC.. but I'd be a bit concerned about breaking things.  I
> guess you could try some sort of kconfig knob to have two "msm"
> devices and see what breaks, but I'm a bit skeptical that we could
> make this the default anytime soon.

Thanks. Yes, breaking userspace would be a bad thing. I do not know if 
we should consider a single GPU+KMS driver to be an ABI and thus set in 
stone.

> 
> For now, just addressing the only-display and only-gpu cases
> (continuing with the single device arrangement when you have both
> display and gpu), maybe split up drm_dev_alloc() and drm_dev_init() so
> that we could use drm_device::driver_features to mask out
> DRIVER_RENDER if needed.

Yep. I'll continue following that path.

> 
> BR,
> -R
> 
>> More on the msm_use_mmu() below.
>>
>>>
>>> Signed-off-by: Jordan Crouse <jorcrous at amazon.com>
>>> ---
>>>
>>>    drivers/gpu/drm/msm/adreno/adreno_device.c | 6 +++++-
>>>    drivers/gpu/drm/msm/msm_drv.c              | 7 +++----
>>>    drivers/gpu/drm/msm/msm_drv.h              | 2 +-
>>>    3 files changed, 9 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
>>> index 36f062c7582f..4f19da28f80f 100644
>>> --- a/drivers/gpu/drm/msm/adreno/adreno_device.c
>>> +++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
>>> @@ -539,7 +539,11 @@ static int adreno_bind(struct device *dev, struct device *master, void *data)
>>>        DBG("Found GPU: %u.%u.%u.%u", config.rev.core, config.rev.major,
>>>                config.rev.minor, config.rev.patchid);
>>>
>>> -     priv->is_a2xx = config.rev.core == 2;
>>> +     /*
>>> +      * A2xx has a built in IOMMU and all other IOMMU enabled targets will
>>> +      * have an ARM IOMMU attached
>>> +      */
>>> +     priv->has_gpu_iommu = config.rev.core == 2 || device_iommu_mapped(dev);
>>>        priv->has_cached_coherent = config.rev.core >= 6;
>>>
>>>        gpu = info->init(drm);
>>> diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
>>> index aca48c868c14..a125a351ec90 100644
>>> --- a/drivers/gpu/drm/msm/msm_drv.c
>>> +++ b/drivers/gpu/drm/msm/msm_drv.c
>>> @@ -318,11 +318,10 @@ bool msm_use_mmu(struct drm_device *dev)
>>>        struct msm_drm_private *priv = dev->dev_private;
>>>
>>>        /*
>>> -      * a2xx comes with its own MMU
>>> -      * On other platforms IOMMU can be declared specified either for the
>>> -      * MDP/DPU device or for its parent, MDSS device.
>>> +      * Return true if the GPU or the MDP/DPU or parent MDSS device has an
>>> +      * IOMMU
>>>         */
>>> -     return priv->is_a2xx ||
>>> +     return priv->has_gpu_iommu ||
>>>                device_iommu_mapped(dev->dev) ||
>>>                device_iommu_mapped(dev->dev->parent);
>>
>> I have a generic feeling that both old an new code is not fully correct.
>> Please correct me if I'm wrong:
>>
>> We should be using VRAM, if either of the blocks can not use remapped
>> memory. So this should have been:
>>
>> bool msm_use_mmu()
>> {
>>    if (!gpu_has_iommu)
>>      return false;
>>
>>    if (have_display_part && !display_has_mmu())
>>      return false;
>>
>>    return true;
>> }
>>
>> What do you think.
>>
>>>    }
>>> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
>>> index 9f0c184b02a0..f33f94acd1b9 100644
>>> --- a/drivers/gpu/drm/msm/msm_drv.h
>>> +++ b/drivers/gpu/drm/msm/msm_drv.h
>>> @@ -126,7 +126,7 @@ struct msm_drm_private {
>>>        struct msm_gpu *gpu;
>>>
>>>        /* gpu is only set on open(), but we need this info earlier */
>>> -     bool is_a2xx;
>>> +     bool has_gpu_iommu;
>>>        bool has_cached_coherent;
>>>
>>>        struct drm_fb_helper *fbdev;
>>
>> --
>> With best wishes
>> Dmitry
>>

-- 
With best wishes
Dmitry