Date:   Thu, 24 Oct 2019 18:57:39 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/3] drm/tegra: Fix 2d and 3d clients detaching from
 IOMMU domain

24.10.2019 18:56, Thierry Reding пишет:
> On Thu, Oct 24, 2019 at 06:47:23PM +0300, Dmitry Osipenko wrote:
>> 24.10.2019 16:50, Thierry Reding пишет:
>>> On Thu, Oct 24, 2019 at 04:28:41PM +0300, Dmitry Osipenko wrote:
>>>> 24.10.2019 14:58, Thierry Reding пишет:
>>>>> On Sun, Jun 23, 2019 at 08:37:42PM +0300, Dmitry Osipenko wrote:
>>>>>> This should should fire up on the DRM's driver module re-loader because
>>>>>> there won't be enough available domains on older Tegra SoCs.
>>>>>>
>>>>>> Cc: stable <stable@...r.kernel.org>
>>>>>> Fixes: 0c407de5ed1a ("drm/tegra: Refactor IOMMU attach/detach")
>>>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>>>> ---
>>>>>>  drivers/gpu/drm/tegra/dc.c   | 4 ++--
>>>>>>  drivers/gpu/drm/tegra/drm.c  | 9 ++++++---
>>>>>>  drivers/gpu/drm/tegra/drm.h  | 3 ++-
>>>>>>  drivers/gpu/drm/tegra/gr2d.c | 4 ++--
>>>>>>  drivers/gpu/drm/tegra/gr3d.c | 4 ++--
>>>>>>  5 files changed, 14 insertions(+), 10 deletions(-)
>>>>>
>>>>> I think I understand what this is trying to do, but the commit message
>>>>> does not help at all. So what's really going on here is that we need to
>>>>> detach the device from the group regardless of whether we're sharing the
>>>>> group or not, just like we attach groups to the shared domain whether
>>>>> they share the same group or not.
>>>>
>>>> Yes, the commit's message could be improved.
>>>>
>>>>> But in that case, I wonder if it's even worth splitting groups the way
>>>>> we are right now. Wouldn't it be better to just put all the devices into
>>>>> the same group and be done with it?
>>>>>
>>>>> The current code gives me headaches every time I read it, so if we can
>>>>> just make it so that all the devices under the DRM device share the same
>>>>> group, this would become a lot easier to deal with. I'm not really
>>>>> convinced that it makes much sense to keep them on separate domains,
>>>>> especially given the constraints on the number of domains available on
>>>>> earlier Tegra devices.
>>>>>
>>>>> Note that sharing a group will also make it much easier for these to use
>>>>> the DMA API if it is backed by an IOMMU.
>>>>
>>>> Probably I'm blanking on everything about IOMMU now.. could you please
>>>> remind me what "IOMMU group" is?
>>>>
>>>> Isn't it that each IOMMU group relates to the HW ID (SWGROUP)? But then
>>>> each display controller has its own SWGROUP.. and thus that sharing just
>>>> doesn't make any sense, hm.
>>>
>>> IOMMU groups are not directly related to SWGROUPs. But by default the
>>> IOMMU framework will share a domain between members of the same IOMMU
>>> group.
>>
>> Ah, I re-figured out that again. The memory controller drivers are
>> defining a single "IOMMU group" for both of the display controllers.
>>
>>> Seems like that's really what we want here, so that when we do
>>> use the DMA API, all the devices part of the DRM device get attached to
>>> the same IOMMU domain, yet if we don't want to use the DMA API we only
>>> need to detach the one group from the backing.
>>
>> Yes, it should be okay to put all DRM devices into the same group, like
>> it is done now for the displays. It also should resolve problem with the
>> domains shortage on T30 since now there are maximum 3 domains in use:
>> host1x, drm and vde.
>>
>> I actually just checked that the original problem still exists
>> and this change solves it as well:
>>
>> ---
>> diff --git a/drivers/memory/tegra/tegra30.c b/drivers/memory/tegra/tegra30.c
>> index 5a0f6e0a1643..e71096498436 100644
>> --- a/drivers/memory/tegra/tegra30.c
>> +++ b/drivers/memory/tegra/tegra30.c
>> @@ -1021,6 +1021,9 @@ static const struct tegra_smmu_swgroup
>> tegra30_swgroups[] = {
>>  static const unsigned int tegra30_group_display[] = {
>>  	TEGRA_SWGROUP_DC,
>>  	TEGRA_SWGROUP_DCB,
>> +	TEGRA_SWGROUP_G2,
>> +	TEGRA_SWGROUP_NV,
>> +	TEGRA_SWGROUP_NV2,
>>  };
>>
>>  static const struct tegra_smmu_group_soc tegra30_groups[] = {
>> ---
>>
>> Please let me know whether you're going to make a patch or if I should
>> do it.
> 
> I've been testing with a similar change and couldn't find any
> regressions. I've also made the same modifications for Tegra114 and
> Tegra124.
> 
> Are you saying that none of these patches are needed anymore? Or do we
> still need a patch to fix detaching? I'm thinking that maybe we can
> drastrically simplify the detachment now by dropping the shared
> parameter altogether.
> 
> Let me draft a patch and send out the whole set for testing.

Seems it's still not ideal because I noticed this in KMSG:

[    0.703185] Failed to attached device 54200000.dc to IOMMU_mapping
[    0.710404] Failed to attached device 54240000.dc to IOMMU_mapping
[    0.719347] Failed to attached device 54140000.gr2d to IOMMU_mapping
[    0.719569] Failed to attached device 54180000.gr3d to IOMMU_mapping

which comes from the implicit IOMMU backing.

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