Re: [RESEND PATCH v2 4/5] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Jonathan Marek <jonathan@marek.ca>]

On 11/14/20 2:39 PM, Rob Clark wrote:
> On Sat, Nov 14, 2020 at 10:58 AM Jonathan Marek <jonathan@marek.ca> wrote:
>>
>> On 11/14/20 1:46 PM, Rob Clark wrote:
>>> On Sat, Nov 14, 2020 at 8:24 AM Christoph Hellwig <hch@lst.de> wrote:
>>>>
>>>> On Sat, Nov 14, 2020 at 10:17:12AM -0500, Jonathan Marek wrote:
>>>>> +void msm_gem_sync_cache(struct drm_gem_object *obj, uint32_t flags,
>>>>> +             size_t range_start, size_t range_end)
>>>>> +{
>>>>> +     struct msm_gem_object *msm_obj = to_msm_bo(obj);
>>>>> +     struct device *dev = msm_obj->base.dev->dev;
>>>>> +
>>>>> +     /* exit early if get_pages() hasn't been called yet */
>>>>> +     if (!msm_obj->pages)
>>>>> +             return;
>>>>> +
>>>>> +     /* TODO: sync only the specified range */
>>>>> +
>>>>> +     if (flags & MSM_GEM_SYNC_FOR_DEVICE) {
>>>>> +             dma_sync_sg_for_device(dev, msm_obj->sgt->sgl,
>>>>> +                             msm_obj->sgt->nents, DMA_TO_DEVICE);
>>>>> +     }
>>>>> +
>>>>> +     if (flags & MSM_GEM_SYNC_FOR_CPU) {
>>>>> +             dma_sync_sg_for_cpu(dev, msm_obj->sgt->sgl,
>>>>> +                             msm_obj->sgt->nents, DMA_FROM_DEVICE);
>>>>> +     }
>>>>
>>>> Splitting this helper from the only caller is rather strange, epecially
>>>> with the two unused arguments.  And I think the way this is specified
>>>> to take a range, but ignoring it is actively dangerous.  User space will
>>>> rely on it syncing everything sooner or later and then you are stuck.
>>>> So just define a sync all primitive for now, and if you really need a
>>>> range sync and have actually implemented it add a new ioctl for that.
>>>
>>> We do already have a split of ioctl "layer" which enforces valid ioctl
>>> params, etc, and gem (or other) module code which is called by the
>>> ioctl func.  So I think it is fine to keep this split here.  (Also, I
>>> think at some point there will be a uring type of ioctl alternative
>>> which would re-use the same gem func.)
>>>
>>> But I do agree that the range should be respected or added later..
>>> drm_ioctl() dispatch is well prepared for extending ioctls.
>>>
>>> And I assume there should be some validation that the range is aligned
>>> to cache-line?  Or can we flush a partial cache line?
>>>
>>
>> The range is intended to be "sync at least this range", so that
>> userspace doesn't have to worry about details like that.
>>
> 
> I don't think userspace can *not* worry about details like that.
> Consider a case where the cpu and gpu are simultaneously accessing
> different parts of a buffer (for ex, sub-allocation).  There needs to
> be cache-line separation between the two.
> 

Right.. and it also seems like we can't get away with just 
flushing/invalidating the whole thing.

qcom's vulkan driver has nonCoherentAtomSize=1, and it looks like 
dma_sync_single_for_cpu() does deal in some way with the partial cache 
line case, although I'm not sure that means we can have a 
nonCoherentAtomSize=1.

> BR,
> -R
>