linux-kernel - Re: [Intel-gfx] [PATCH] drm/i915/display: Fix phys

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Date:   Tue, 28 Nov 2023 12:12:08 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Paz Zcharya <pazz@...omium.org>
Cc:     Subrata Banik <subratabanik@...gle.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        intel-gfx@...ts.freedesktop.org,
        Marcin Wojtas <mwojtas@...omium.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Sean Paul <seanpaul@...omium.org>, matthew.auld@...el.com,
        Daniel Vetter <daniel@...ll.ch>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Drew Davenport <ddavenport@...omium.org>,
        David Airlie <airlied@...il.com>,
        Nirmoy Das <nirmoy.das@...el.com>
Subject: Re: [Intel-gfx] [PATCH] drm/i915/display: Fix phys_base to be
 relative not absolute

On 28.11.2023 04:47, Paz Zcharya wrote:
> 
> On Mon, Nov 27, 2023 at 8:20 PM Paz Zcharya <pazz@...omium.org> wrote:
>>
>> On 21.11.2023 13:06, Andrzej Hajda wrote:
>>> On 18.11.2023 00:01, Paz Zcharya wrote:
>>>> On Tue, Nov 14, 2023 at 10:13:59PM -0500, Rodrigo Vivi wrote:
>>>>> On Sun, Nov 05, 2023 at 05:27:03PM +0000, Paz Zcharya wrote:
>>>>
>>>> Hi Rodrigo, thanks for the great comments.
>>>>
>>>> Apologies for using a wrong/confusing terminology. I think 'phys_base'
>>>> is supposed to be the offset in the GEM BO, where base (or
>>>> "Surface Base Address") is supposed to be the GTT offset.
>>>
>>> Since base is taken from PLANE_SURF register it should be resolvable via
>>> GGTT to physical address pointing to actual framebuffer.
>>> I couldn't find anything in the specs.
>>
>> It was quite cryptic. I meant I have not found anything about assumption
>> from commit history that for iGPU there should be 1:1 mapping, this is why
>> there was an assignment "phys_base = base". Possibly the assumption is not
>> valid anymore for MTL(?).
>> Without the assumption we need to check GGTT to determine phys address.
>>
>>> The simplest approach would be then do the same as in case of DGFX:
>>>           gen8_pte_t __iomem *gte = to_gt(i915)->ggtt->gsm;
>>>           gen8_pte_t pte;
>>>
>>>           gte += base / I915_GTT_PAGE_SIZE;
>>>
>>>           pte = ioread64(gte);
>>>           phys_base = pte & I915_GTT_PAGE_MASK;
>>>
>>> Regards
>>> Andrzej
> 
> Hey Andrzej,
> 
> On a second thought, what do you think about something like
> 
> +               gen8_pte_t __iomem *gte = to_gt(i915)->ggtt->gsm;
> +               gen8_pte_t pte;
> +               gte += base / I915_GTT_PAGE_SIZE;
> +               pte = ioread64(gte);
> +               pte = pte & I915_GTT_PAGE_MASK;
> +               phys_base = pte - i915->mm.stolen_region->region.start;
> 
> The only difference is the last line.

Bingo :) It seems to be generic algorithm to get phys_base for all 
platforms:
- on older platforms stolen_region points to system memory which starts 
at 0,
- on DG2 it uses lmem region which starts at 0 as well,
- on MTL stolen_region points to stolen-local which starts at 0x800000.

So this whole "if (IS_DGFX(i915)) {...} else {...}" could be replaced
with sth generic.
1. Find pte.
2. if(IS_DGFX(i915) && pte & GEN12_GGTT_PTE_LM) mem = 
i915->mm.regions[INTEL_REGION_LMEM_0] else mem = i915->mm.stolen_region
3. phys_base = (pte & I915_GTT_PAGE_MASK) - mem->region.start;

Regards
Andrzej


> 
> Based on what I wrote before, I think `phys_base` is named incorrectly and
> that it does not reflect the physical address, but the start offset of
> i915->mm.stolen_region. So if we offset the start value of the stolen
> region, this code looks correct to me (and it also works on my
> MeteorLake device).
> 
> What do you think?
> 
> 
> Many thanks,
> Paz
>