Message-ID: <7f7ecf90-531c-69ae-9011-684666ed8743@gmail.com>
Date:   Thu, 24 Oct 2019 19:09:45 +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 18:57, Dmitry Osipenko пишет:
> 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.

And the error comes from here:

https://elixir.bootlin.com/linux/v5.4-rc2/source/drivers/iommu/iommu.c#L1655

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