linux-kernel - Re: [RFC PATCH 5/7] drm/ttm: add range busy check for range manager

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Date:   Wed, 16 Mar 2022 15:39:24 +0100
From:   Christian König <christian.koenig@....com>
To:     Robert Beckett <bob.beckett@...labora.com>,
        intel-gfx@...ts.freedesktop.org, Huang Rui <ray.huang@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/7] drm/ttm: add range busy check for range manager

Am 16.03.22 um 15:26 schrieb Robert Beckett:
>
> [SNIP]
> this is where I replace an existing range check via drm_mm with the 
> range check I added in this patch.

Mhm, I still don't get the use case from the code, but I don't think it 
matters any more.

>>> I suppose we could add another drm_mm range tracker just for testing 
>>> and shadow track each allocation in the range, but that seemed like 
>>> a lot of extra infrastructure for no general runtime use.
>>
>> I have no idea what you mean with that.
>
> I meant as a potential solution to tracking allocations without a 
> range check, we would need to add something external. e.g. adding a 
> shadow drm_mm range tracker, or a bitmask across the range, or stick 
> objects in a list etc.

Ah! So you are trying to get access to the drm_mm inside the 
ttm_range_manager and not add some additional range check function! Now 
I got your use case.

>>> would you mind explaining the rationale for removing range checks? 
>>> It seems to me like a natural fit for a memory manager
>>
>> TTM manages buffer objects and resources, not address space. The 
>> lpfn/fpfn parameter for the resource allocators are actually used as 
>> just two independent parameters and not define any range. We just 
>> keep the names for historical reasons.
>>
>> The only places we still use and compare them as ranges are 
>> ttm_resource_compat() and ttm_bo_eviction_valuable() and I already 
>> have patches to clean up those and move them into the backend 
>> resource handling.
>
> except the ttm_range_manager seems to still use them as a range specifier.

Yeah, because the range manager is the backend which handles ranges 
using the drm_mm :)

> If the general design going forward is to not consider ranges, how 
> would you recommend constructing buffers around pre-allocated regions 
> e.g. uefi frame buffers who's range is dictated externally?

Call ttm_bo_mem_space() with the fpfn/lpfn filled in as required. See 
function amdgpu_bo_create_kernel_at() for an example.

Regards,
Christian.

>
>>
>> Regards,
>> Christian.
>>
>>>
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>>
>>>>> Signed-off-by: Robert Beckett <bob.beckett@...labora.com>
>>>>> ---
>>>>>   drivers/gpu/drm/ttm/ttm_range_manager.c | 21 +++++++++++++++++++++
>>>>>   include/drm/ttm/ttm_range_manager.h     |  3 +++
>>>>>   2 files changed, 24 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_range_manager.c 
>>>>> b/drivers/gpu/drm/ttm/ttm_range_manager.c
>>>>> index 8cd4f3fb9f79..5662627bb933 100644
>>>>> --- a/drivers/gpu/drm/ttm/ttm_range_manager.c
>>>>> +++ b/drivers/gpu/drm/ttm/ttm_range_manager.c
>>>>> @@ -206,3 +206,24 @@ int ttm_range_man_fini_nocheck(struct 
>>>>> ttm_device *bdev,
>>>>>       return 0;
>>>>>   }
>>>>>   EXPORT_SYMBOL(ttm_range_man_fini_nocheck);
>>>>> +
>>>>> +/**
>>>>> + * ttm_range_man_range_busy - Check whether anything is allocated 
>>>>> with a range
>>>>> + *
>>>>> + * @man: memory manager to check
>>>>> + * @fpfn: first page number to check
>>>>> + * @lpfn: last page number to check
>>>>> + *
>>>>> + * Return: true if anything allocated within the range, false 
>>>>> otherwise.
>>>>> + */
>>>>> +bool ttm_range_man_range_busy(struct ttm_resource_manager *man,
>>>>> +                  unsigned fpfn, unsigned lpfn)
>>>>> +{
>>>>> +    struct ttm_range_manager *rman = to_range_manager(man);
>>>>> +    struct drm_mm *mm = &rman->mm;
>>>>> +
>>>>> +    if (__drm_mm_interval_first(mm, PFN_PHYS(fpfn), PFN_PHYS(lpfn 
>>>>> + 1) - 1))
>>>>> +        return true;
>>>>> +    return false;
>>>>> +}
>>>>> +EXPORT_SYMBOL(ttm_range_man_range_busy);
>>>>> diff --git a/include/drm/ttm/ttm_range_manager.h 
>>>>> b/include/drm/ttm/ttm_range_manager.h
>>>>> index 7963b957e9ef..86794a3f9101 100644
>>>>> --- a/include/drm/ttm/ttm_range_manager.h
>>>>> +++ b/include/drm/ttm/ttm_range_manager.h
>>>>> @@ -53,4 +53,7 @@ static __always_inline int 
>>>>> ttm_range_man_fini(struct ttm_device *bdev,
>>>>>       BUILD_BUG_ON(__builtin_constant_p(type) && type >= 
>>>>> TTM_NUM_MEM_TYPES);
>>>>>       return ttm_range_man_fini_nocheck(bdev, type);
>>>>>   }
>>>>> +
>>>>> +bool ttm_range_man_range_busy(struct ttm_resource_manager *man,
>>>>> +                  unsigned fpfn, unsigned lpfn);
>>>>>   #endif
>>>>
>>