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

[Dmitry Osipenko]

26.03.2021 19:37, Thierry Reding пишет:
> On Wed, Mar 24, 2021 at 07:50:01PM +0300, Dmitry Osipenko wrote:
>> 24.03.2021 19:42, Thierry Reding пишет:
>>> 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 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?
>>>
>>> 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.
>>
>> Isn't it really possible to put displays into the same IOMMU group on
>> T194? It doesn't make much sense to have them in a separate groups on Linux.
> 
> It is possible and in fact that's what's already happening. However, the
> problem isn't that these devices are not in the same group, the problem
> is that the DMA API doesn't know anything about groups. It works on
> struct device and if you've got DMA API debugging enabled it may even
> flag incorrect usage as errors.
> 
> So from a DMA API point of view, if a device wants to use a buffer, that
> buffer first has to be mapped for that device, even if it was already
> mapped for a different device that happens to be in the same IOMMU group
> and hence share an IOMMU domain.

This sounds to me like something which needs to be addressed first, i.e.
to make DMA API aware that it's okay to re-use mappings by sibling
devices within the same IOMMU group. Although, I assume that you already
considered this variant, didn't you?