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

[Thierry Reding <thierry.reding@gmail.com>]

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On Wed, Mar 24, 2021 at 06:45:30PM +0300, Dmitry Osipenko wrote:
> 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@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@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?

I suppose that would be possible. However, tegra_fb_create() doesn't
know a thing about display controllers, so we'd have to add extra code
to it to iterate over all display controllers and do a dma_map_sg() of
the GEM object for each of them.

It's also somewhat wasteful because now we get a mapping for each
framebuffer for each display controller. So if you've got, say, a four
UHD screen setup (which is something that Tegra194 supports), you could
end up with 8 UHD framebuffers (two for each display, for double-
buffering) at 32 MiB each for a whopping 256 MiB of memory that needs to
be mapped for each of the four display controllers. That 1 GiB worth of
page table updates, whereas you really only need one fourth of that.

Granted, this will make flipping a bit faster, and IOVA space isn't
really a problem on Tegra194. It would still waste a bit of RAM for all
those page table entries that we don't really need, though.

A mapping cache seems like a much better compromise because the cache
lookup should be quite fast compared to a mapping operation and we waste
just a couple dozen bytes per mapping perhaps as opposed to a few
megabytes for the gratuitous, preemptive mappings.

Thierry

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