[PATCH 6/9] drm/tegra: gem: Add a clarifying comment

[Dmitry Osipenko]

24.03.2021 18:02, Thierry Reding пишет:
> On Wed, Mar 24, 2021 at 05:41:08PM +0300, Dmitry Osipenko wrote:
>> 23.03.2021 18:54, Thierry Reding пишет:
>>> From: Thierry Reding <treding at nvidia.com>
>>>
>>> Clarify when a fixed IOV address can be used and when a buffer has to
>>> be mapped before the IOVA can be used.
>>>
>>> Signed-off-by: Thierry Reding <treding at nvidia.com>
>>> ---
>>>  drivers/gpu/drm/tegra/plane.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/tegra/plane.c b/drivers/gpu/drm/tegra/plane.c
>>> index 19e8847a164b..793da5d675d2 100644
>>> --- a/drivers/gpu/drm/tegra/plane.c
>>> +++ b/drivers/gpu/drm/tegra/plane.c
>>> @@ -119,6 +119,14 @@ static int tegra_dc_pin(struct tegra_dc *dc, struct tegra_plane_state *state)
>>>  		dma_addr_t phys_addr, *phys;
>>>  		struct sg_table *sgt;
>>>  
>>> +		/*
>>> +		 * If we're not attached to a domain, we already stored the
>>> +		 * physical address when the buffer was allocated. If we're
>>> +		 * part of a group that's shared between all display
>>> +		 * controllers, we've also already mapped the framebuffer
>>> +		 * through the SMMU. In both cases we can short-circuit the
>>> +		 * code below and retrieve the stored IOV address.
>>> +		 */
>>>  		if (!domain || dc->client.group)
>>>  			phys = &phys_addr;
>>>  		else
>>>
>>
>> This comment is correct, but the logic feels a bit lame because it
>> should be wasteful to re-map DMA on each FB flip. Personally I don't
>> care much about this since older Tegras use pinned buffers by default,
>> but this shouldn't be good for T124+ users.
> 
> I'm not terribly thrilled by this either, but it's the only way to do
> this when using the DMA API because we don't know at allocation time (or
> import time for that matter) which of the (up to) 4 display controllers
> a framebuffer will be shown on. tegra_dc_pin() is the earliest where
> this is known and worst case that's called once per flip.
> 
> When the IOMMU API is used explicitly, we always map framebuffers into
> the IOMMU domain shared by all display controllers at allocation or
> import time and then we don't need to pin at flip time anymore.
> 
> I do have a work-in-progress patch somewhere that creates a mapping
> cache to mitigate this problem to some degree. I need to dig that up and
> do a few measurements because I vaguely recall this speeding up flips by
> quite a bit (well, except for the very first mapping, obviously).
> 
>> Perhaps dumb buffers should be pinned to display by default and then we
>> should extend the Tegra UAPI to support BO mapping to display client(?).
> 
> That would kind of defeat the purpose of a generic KMS UAPI.

Couldn't the BOs be mapped when FB is created, i.e. by tegra_fb_create?