Re: [Intel-gfx] [PATCH 7/8] drm/i915: fixup the initial fb base on DG1

[Matthew Auld <matthew.auld@intel.com>]

On 07/03/2022 17:06, Ville Syrjälä wrote:
> On Mon, Mar 07, 2022 at 10:32:36AM +0000, Matthew Auld wrote:
>> On 04/03/2022 19:33, Ville Syrjälä wrote:
>>> On Fri, Mar 04, 2022 at 05:23:32PM +0000, Matthew Auld wrote:
>>>> The offset we get looks to be the exact start of DSM, but the
>>>> inital_plane_vma expects the address to be relative.
>>>>
>>>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>>>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>>> ---
>>>>    .../drm/i915/display/intel_plane_initial.c    | 22 +++++++++++++++----
>>>>    1 file changed, 18 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/display/intel_plane_initial.c b/drivers/gpu/drm/i915/display/intel_plane_initial.c
>>>> index f797fcef18fc..b39d3a8dfe45 100644
>>>> --- a/drivers/gpu/drm/i915/display/intel_plane_initial.c
>>>> +++ b/drivers/gpu/drm/i915/display/intel_plane_initial.c
>>>> @@ -56,10 +56,24 @@ initial_plane_vma(struct drm_i915_private *i915,
>>>>    	if (!mem || plane_config->size == 0)
>>>>    		return NULL;
>>>>    
>>>> -	base = round_down(plane_config->base,
>>>> -			  I915_GTT_MIN_ALIGNMENT);
>>>> -	size = round_up(plane_config->base + plane_config->size,
>>>> -			mem->min_page_size);
>>>> +	base = plane_config->base;
>>>> +	if (IS_DGFX(i915)) {
>>>> +		/*
>>>> +		 * On discrete the base address should be somewhere in LMEM, but
>>>> +		 * depending on the size of LMEM the base address might
>>>> +		 * intersect with the start of DSM, like on DG1, in which case
>>>> +		 * we need the relative address. In such cases we might also
>>>> +		 * need to choose between inital fb vs fbc, if space is limited.
>>>> +		 *
>>>> +		 * On future discrete HW, like DG2, we should be able to just
>>>> +		 * allocate directly from LMEM, due to larger LMEM size.
>>>> +		 */
>>>> +		if (base >= i915->dsm.start)
>>>> +			base -= i915->dsm.start;
>>>
>>> Subsequent code expects the object to actually be inside stolen.
>>> If that is not the case we should just give up.
>>
>> Thanks for taking a look at this. Is that subsequent code outside
>> initial_plane_vma()? In the next patch this is now using LMEM directly
>> for dg2. Would that blow up somewhere else?
> 
> It uses i915_gem_object_create_stolen_for_preallocated() which assumes
> the stuff is inside stolen.

At the start of the series that gets ripped out and replaced with 
i915_gem_object_create_region_at(), where we can now just pass in the 
intel_memory_region, and the backend hopefully takes care of the rest.

> 
>>> The fact that we fail to confirm any of that on integrated
>>> parts has always bugged me, but not enough to actually do
>>> anything about it. Such a check would be somewhat more involved
>>> since we'd have to look at the PTEs. But on discrete sounds like
>>> we can get away with a trivial check.
>>
>> Which PTEs?
> 
> The PTEs the plane is actually using. We have no idea where they
> actually point to and just assume they represent a 1:1 mapping of
> stolen.
> 
> I suppose with lmem we'll just start assuming a 1:1 mapping of
> the whole lmem rather than just stolen.

So IIUC the base that we read is actually some GGTT address(I guess it 
comes pre-programmed or something?), and that hopefully 1:1 maps to 
stolen. Ok, so as you say, I guess we only want to subtract the 
dsm.start for the physical allocation, and not the GGTT address, when 
dealing with stolen lmem.

>