[PATCH 0/2] iommu: handle drivers that manage iommu directly

[PATCH 0/2] iommu: handle drivers that manage iommu directly

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

One of the challenges we need to handle to enable the aarch64 laptops
upstream is dealing with the fact that the bootloader enables the
display and takes the corresponding SMMU context-bank out of BYPASS.
Unfortunately, currently, the IOMMU framework attaches a DMA (or
potentially an IDENTITY) domain before the driver is probed and has
a chance to intervene and shutdown[1] scanout.  Which makes things go
horribly wrong.

This also happens to solve a problem that is blocking us from supporting
per-context pagetables on the GPU, due to domain that is attached before
driver has a chance to attach it's own domain for the GPU.

But since the driver is managing it's own iommu domains directly, and
does not use dev->iommu_group->default_domain at all, the simple
solution to both problems is to just avoid attaching that domain in the
first place.

[1] Eventually we want to be able to do a seemless transition from
    efifb to drm/msm... but first step is to get the core (iommu,
    clk, genpd) pieces in place, so a first step of disabling the
    display before first modeset enables us to get all of the
    dependencies outside of drm/msm in place.  And this at least
    gets us parity with windows (which also appears to do a modeset
    between bootloader and HLSO).  After that there is a bunch of
    drm/msm work that is probably not interesting to folks outside
    of dri-devel.

Rob Clark (2):
  iommu: add support for drivers that manage iommu explicitly
  drm/msm: mark devices where iommu is managed by driver

 drivers/gpu/drm/msm/adreno/adreno_device.c |  1 +
 drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c    |  1 +
 drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c   |  1 +
 drivers/gpu/drm/msm/msm_drv.c              |  1 +
 drivers/iommu/iommu.c                      | 11 +++++++++++
 include/linux/device.h                     |  3 ++-
 6 files changed, 17 insertions(+), 1 deletion(-)

-- 
2.20.1

[PATCH 2/2] drm/msm: mark devices where iommu is managed by driver

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/msm/adreno/adreno_device.c | 1 +
 drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c    | 1 +
 drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c   | 1 +
 drivers/gpu/drm/msm/msm_drv.c              | 1 +
 4 files changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
index b907245d3d96..d9ac8c4cd866 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_device.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
@@ -441,6 +441,7 @@ static struct platform_driver adreno_driver = {
 		.name = "adreno",
 		.of_match_table = dt_match,
 		.pm = &adreno_pm_ops,
+		.driver_manages_iommu = true,
 	},
 };
 
diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
index 39378ace57a6..001fa7986f31 100644
--- a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
+++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
@@ -1127,6 +1127,7 @@ static struct platform_driver dpu_driver = {
 		.name = "msm_dpu",
 		.of_match_table = dpu_dt_match,
 		.pm = &dpu_pm_ops,
+		.driver_manages_iommu = true,
 	},
 };
 
diff --git a/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c b/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c
index 97179bec8902..2a1b8709d0dc 100644
--- a/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c
+++ b/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c
@@ -1100,6 +1100,7 @@ static struct platform_driver mdp5_driver = {
 		.name = "msm_mdp",
 		.of_match_table = mdp5_dt_match,
 		.pm = &mdp5_pm_ops,
+		.driver_manages_iommu = true,
 	},
 };
 
diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
index 31deb87abfc6..16094b8c5418 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -1352,6 +1352,7 @@ static struct platform_driver msm_platform_driver = {
 		.name   = "msm",
 		.of_match_table = dt_match,
 		.pm     = &msm_pm_ops,
+		.driver_manages_iommu = true,
 	},
 };
 
-- 
2.20.1

Re: [PATCH 1/2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Wed, Jul 3, 2019 at 5:42 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 02/07/2019 21:26, Rob Clark wrote:
> > From: Rob Clark <robdclark@chromium.org>
> >
> > Avoid attaching any non-driver managed domain if the driver indicates
> > that it manages the iommu directly.
> >
> > This solves a couple problems that drm/msm + arm-smmu has with the iommu
> > framework:
> >
> > 1) In some cases the bootloader takes the iommu out of bypass and
> >     enables the display.  This is in particular a problem on the aarch64
> >     laptops that exist these days, and modern snapdragon android devices.
> >     (Older devices also enabled the display in bootloader but did not
> >     take the iommu out of bypass.)  Attaching a DMA or IDENTITY domain
> >     while scanout is active, before the driver has a chance to intervene,
> >     makes things go *boom*
>
> In the general case, we have to assume that things already went boom
> long ago, as soon as the IOMMU itself was probed and reset. By the time
> we get to the point of binding of a client driver, also assume that the
> IOMMU is already powered off and stopping traffic because the RPM device
> links aren't in place yet and it believes itself unused.

you are correct that this is only part of what is needed to get things
working.  We also need Bjorn's patch set to inherit SMR and CB config
during init:

https://www.spinics.net/lists/arm-kernel/msg732246.html

>
> > 2) We are currently blocked on landing support for GPU per-context
> >     pagetables because of the domain attached before driver's ->probe()
> >     is called.
>
> I'm getting a little fed up of explaining that that problem is specific
> to the current behaviour of one particular IOMMU driver and trying to
> work around it anywhere other than in that driver is at best an
> unreliable hack.

Perhaps the GPU part of the problem.  The display part is not.
However I'm fine to move the "don't actually attach" part into
arm-smmu if that is preferred.  The next person to hit the same
problem on a different iommu could certainly move the check or copy it
into their iommu driver.

> > This solves both problems.
>
> For a very, very specific value of "solve"... ;)

well, "solve" == "it boots fine and doesn't explode"..

I'm certainly happy to entertain alternative suggestions, but these
are real problems that need solutions.

BR,
-R

> > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > ---
> >   drivers/iommu/iommu.c  | 11 +++++++++++
> >   include/linux/device.h |  3 ++-
> >   2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 0c674d80c37f..efa0957f9772 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -1573,6 +1573,17 @@ static int __iommu_attach_device(struct iommu_domain *domain,
> >           domain->ops->is_attach_deferred(domain, dev))
> >               return 0;
> >
> > +     /*
> > +      * If driver is going to manage iommu directly, then avoid
> > +      * attaching any non driver managed domain.  There could
> > +      * be already active dma underway (ie. scanout in case of
> > +      * bootloader enabled display), and interfering with that
> > +      * will make things go *boom*
> > +      */
> > +     if ((domain->type != IOMMU_DOMAIN_UNMANAGED) &&
> > +         dev->driver && dev->driver->driver_manages_iommu)
> > +             return 0;
>
> This leaving things half-hanging is really ugly, but more than that it
> assumes that allocating a default domain in the first place isn't
> disruptive - I'm not 100% sure that's *always* the case today, and it's
> definitely likely to change in future as part of improving the current
> request_dm_for_dev() mechanism. As it happens, those proposed changes
> would not only break this idea, but make it redundant, since they're
> about forcing the default domain type to passthrough on a per-device
> basis, which leads to an equivalent end result to this patch, but in a
> cleaner and more robust manner.
>
> Robin.
>
> > +
> >       if (unlikely(domain->ops->attach_dev == NULL))
> >               return -ENODEV;
> >
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index e138baabe01e..d98aa4d3c8c3 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -282,7 +282,8 @@ struct device_driver {
> >       struct module           *owner;
> >       const char              *mod_name;      /* used for built-in modules */
> >
> > -     bool suppress_bind_attrs;       /* disables bind/unbind via sysfs */
> > +     bool suppress_bind_attrs:1;     /* disables bind/unbind via sysfs */
> > +     bool driver_manages_iommu:1;    /* driver manages IOMMU explicitly */
> >       enum probe_type probe_type;
> >
> >       const struct of_device_id       *of_match_table;
> >

Re: [PATCH 1/2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Thu, Jul 4, 2019 at 1:20 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> Hi Rob,
>
> On Tue, Jul 02, 2019 at 01:26:18PM -0700, Rob Clark wrote:
> > 1) In some cases the bootloader takes the iommu out of bypass and
> >    enables the display.  This is in particular a problem on the aarch64
> >    laptops that exist these days, and modern snapdragon android devices.
> >    (Older devices also enabled the display in bootloader but did not
> >    take the iommu out of bypass.)  Attaching a DMA or IDENTITY domain
> >    while scanout is active, before the driver has a chance to intervene,
> >    makes things go *boom*
>
> Just to make sure I get this right: The bootloader inializes the SMMU
> and creates non-identity mappings for the GPU? And when the SMMU driver
> in Linux takes over this breaks display output.

correct

> > +     /*
> > +      * If driver is going to manage iommu directly, then avoid
> > +      * attaching any non driver managed domain.  There could
> > +      * be already active dma underway (ie. scanout in case of
> > +      * bootloader enabled display), and interfering with that
> > +      * will make things go *boom*
> > +      */
> > +     if ((domain->type != IOMMU_DOMAIN_UNMANAGED) &&
> > +         dev->driver && dev->driver->driver_manages_iommu)
> > +             return 0;
> > +
>
> When the default domain is attached, there is usually no driver attached
> yet. I think this needs to be communicated by the firmware to Linux and
> the code should check against that.

At least for the OF case, it happens in the of_dma_configure() which
happens from really_probe(), so there is normally a driver.  There are
a few exceptional cases, where drivers call of_dma_configure() on
their own sub-device without a driver attached (hence the need to
check if dev->driver is NULL).

I'm also interested in the ACPI case eventually... the aarch64
"windows" laptops do have ACPI.  But for now we are booting with DT
since there is quite a lot of work before we get to point of using
ACPI.  (In particular, under windows, device power management is done
thru a Platform Extension  Plugin (PEP), but so far linux has no such
mechanism.)

We really don't have control of the firmware.  But when arm-smmu is
probed it can read back the hw state and figure out what is going on
(with an RFC series[1] from Bjorn which was posted earlier), so we
don't really need to depend on the firmware.

> > -     bool suppress_bind_attrs;       /* disables bind/unbind via sysfs */
> > +     bool suppress_bind_attrs:1;     /* disables bind/unbind via sysfs */
> > +     bool driver_manages_iommu:1;    /* driver manages IOMMU explicitly */
>
> How does this field get set?


It is set in the driver in the second patch[2] in this series.

BR,
-R

[1] https://www.spinics.net/lists/arm-kernel/msg732246.html
[2] https://patchwork.freedesktop.org/patch/315291/

[PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

From: Rob Clark <robdclark@chromium.org>

Avoid attaching any non-driver managed domain if the driver indicates
that it manages the iommu directly.

This avoids a problem on devices where the bootloader takes the SMMU out
of bypass and enables scanout, such as is the case on snapdragon aarch64
laptops and newer snapdragon android devices.  Attaching an IDENTITY or
DMA domain before the driver has a chance to intervene will break efifb
scanout and start triggering iommu faults.

If the driver manages the iommu directly (as does drm/msm), it can
shut down scanout when it is ready to take over the display, before
attaching an UNMANAGED domain.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
v2. Move the check into arm_smmu_attach_dev() (as I *think* this is
    what Robin preferred; update commit msg to focus on the display
    related issue that this solves.

We also need Bjorn's patch set to inherit SMR and CB config during
init:

https://www.spinics.net/lists/arm-kernel/msg732246.html

 drivers/iommu/arm-smmu.c | 11 +++++++++++
 include/linux/device.h   |  3 ++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 1a5efa7c8767..4a80710124db 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -1411,6 +1411,17 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
 		return -ENXIO;
 	}
 
+	/*
+	 * If driver is going to manage iommu directly, then avoid
+	 * attaching any non driver managed domain.  There could
+	 * be already active dma underway (ie. scanout in case of
+	 * bootloader enabled display), and interfering with that
+	 * will make things go *boom*
+	 */
+	if ((domain->type != IOMMU_DOMAIN_UNMANAGED) &&
+	    dev->driver && dev->driver->driver_manages_iommu)
+		return 0;
+
 	/*
 	 * FIXME: The arch/arm DMA API code tries to attach devices to its own
 	 * domains between of_xlate() and add_device() - we have no way to cope
diff --git a/include/linux/device.h b/include/linux/device.h
index e138baabe01e..d98aa4d3c8c3 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -282,7 +282,8 @@ struct device_driver {
 	struct module		*owner;
 	const char		*mod_name;	/* used for built-in modules */
 
-	bool suppress_bind_attrs;	/* disables bind/unbind via sysfs */
+	bool suppress_bind_attrs:1;	/* disables bind/unbind via sysfs */
+	bool driver_manages_iommu:1;	/* driver manages IOMMU explicitly */
 	enum probe_type probe_type;
 
 	const struct of_device_id	*of_match_table;
-- 
2.20.1

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Joerg Roedel]

On Wed, Jul 10, 2019 at 11:28:30AM -0700, Rob Clark wrote:
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -282,7 +282,8 @@ struct device_driver {
>  	struct module		*owner;
>  	const char		*mod_name;	/* used for built-in modules */
>  
> -	bool suppress_bind_attrs;	/* disables bind/unbind via sysfs */
> +	bool suppress_bind_attrs:1;	/* disables bind/unbind via sysfs */
> +	bool driver_manages_iommu:1;	/* driver manages IOMMU explicitly */

Who will set this bit?


Regards,

	Joerg

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Mon, Jul 22, 2019 at 7:28 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> On Wed, Jul 10, 2019 at 11:28:30AM -0700, Rob Clark wrote:
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -282,7 +282,8 @@ struct device_driver {
> >       struct module           *owner;
> >       const char              *mod_name;      /* used for built-in modules */
> >
> > -     bool suppress_bind_attrs;       /* disables bind/unbind via sysfs */
> > +     bool suppress_bind_attrs:1;     /* disables bind/unbind via sysfs */
> > +     bool driver_manages_iommu:1;    /* driver manages IOMMU explicitly */
>
> Who will set this bit?
>

It is set by the driver:

https://patchwork.freedesktop.org/patch/315291/

(This doesn't really belong in devicetree, since it isn't a
description of the hardware, so the driver is really the only place to
set this.. which is fine because it is about a detail of how the
driver works.)

BR,
-R

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Joerg Roedel]

On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> It is set by the driver:
> 
> https://patchwork.freedesktop.org/patch/315291/
> 
> (This doesn't really belong in devicetree, since it isn't a
> description of the hardware, so the driver is really the only place to
> set this.. which is fine because it is about a detail of how the
> driver works.)

It is more a detail about how the firmware works. IIUC the problem is
that the firmware initializes the context mappings for the GPU and the
OS doesn't know anything about that and just overwrites them, causing
the firmware GPU driver to fail badly.

So I think it is the task of the firmware to tell the OS not to touch
the devices mappings until the OS device driver takes over. On x86 there
is something similar with the RMRR/unity-map tables from the firmware.

Regards,

	Joerg

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > It is set by the driver:
> >
> > https://patchwork.freedesktop.org/patch/315291/
> >
> > (This doesn't really belong in devicetree, since it isn't a
> > description of the hardware, so the driver is really the only place to
> > set this.. which is fine because it is about a detail of how the
> > driver works.)
>
> It is more a detail about how the firmware works. IIUC the problem is
> that the firmware initializes the context mappings for the GPU and the
> OS doesn't know anything about that and just overwrites them, causing
> the firmware GPU driver to fail badly.
>
> So I think it is the task of the firmware to tell the OS not to touch
> the devices mappings until the OS device driver takes over. On x86 there
> is something similar with the RMRR/unity-map tables from the firmware.
>

Bjorn had a patchset[1] to inherit the config from firmware/bootloader
when arm-smmu is probed which handles that part of the problem.  My
patch is intended to be used on top of his patchset.  This seems to me
like the best solution, if we don't have control over the firmware.

BR,
-R

[1] https://www.spinics.net/lists/arm-kernel/msg732246.html

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Will Deacon]

On Mon, Jul 22, 2019 at 09:23:48AM -0700, Rob Clark wrote:
> On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
> >
> > On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > > It is set by the driver:
> > >
> > > https://patchwork.freedesktop.org/patch/315291/
> > >
> > > (This doesn't really belong in devicetree, since it isn't a
> > > description of the hardware, so the driver is really the only place to
> > > set this.. which is fine because it is about a detail of how the
> > > driver works.)
> >
> > It is more a detail about how the firmware works. IIUC the problem is
> > that the firmware initializes the context mappings for the GPU and the
> > OS doesn't know anything about that and just overwrites them, causing
> > the firmware GPU driver to fail badly.
> >
> > So I think it is the task of the firmware to tell the OS not to touch
> > the devices mappings until the OS device driver takes over. On x86 there
> > is something similar with the RMRR/unity-map tables from the firmware.
> >
> 
> Bjorn had a patchset[1] to inherit the config from firmware/bootloader
> when arm-smmu is probed which handles that part of the problem.  My
> patch is intended to be used on top of his patchset.  This seems to me
> like the best solution, if we don't have control over the firmware.

Hmm, but the feedback from Robin on the thread you cite was that this should
be generalised to look more like RMRR, so there seems to be a clear message
here.

Will

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Tue, Jul 23, 2019 at 8:38 AM Will Deacon <will@kernel.org> wrote:
>
> On Mon, Jul 22, 2019 at 09:23:48AM -0700, Rob Clark wrote:
> > On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
> > >
> > > On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > > > It is set by the driver:
> > > >
> > > > https://patchwork.freedesktop.org/patch/315291/
> > > >
> > > > (This doesn't really belong in devicetree, since it isn't a
> > > > description of the hardware, so the driver is really the only place to
> > > > set this.. which is fine because it is about a detail of how the
> > > > driver works.)
> > >
> > > It is more a detail about how the firmware works. IIUC the problem is
> > > that the firmware initializes the context mappings for the GPU and the
> > > OS doesn't know anything about that and just overwrites them, causing
> > > the firmware GPU driver to fail badly.
> > >
> > > So I think it is the task of the firmware to tell the OS not to touch
> > > the devices mappings until the OS device driver takes over. On x86 there
> > > is something similar with the RMRR/unity-map tables from the firmware.
> > >
> >
> > Bjorn had a patchset[1] to inherit the config from firmware/bootloader
> > when arm-smmu is probed which handles that part of the problem.  My
> > patch is intended to be used on top of his patchset.  This seems to me
> > like the best solution, if we don't have control over the firmware.
>
> Hmm, but the feedback from Robin on the thread you cite was that this should
> be generalised to look more like RMRR, so there seems to be a clear message
> here.
>

Perhaps it is a lack of creativity, or lack of familiarity w/ iommu vs
virtualization, but I'm not quite seeing how RMRR would help.. in
particular when dealing with both DT and ACPI cases.  So I kinda
prefer, when possible, if arm-smmu can figure out what is going on by
looking at the hw state at boot (since that approach would work
equally well for DT and ACPI).

I *think* (but need to confirm if Bjorn hasn't already) that the
memory for the pagetables that firmware/bootloader sets up is already
removed from the memory map efi passes to kernel, so we don't need to
worry about kernel stomping in-use pagetables.

BR,
-R

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Will Deacon]

On Tue, Jul 23, 2019 at 10:40:55AM -0700, Rob Clark wrote:
> On Tue, Jul 23, 2019 at 8:38 AM Will Deacon <will@kernel.org> wrote:
> >
> > On Mon, Jul 22, 2019 at 09:23:48AM -0700, Rob Clark wrote:
> > > On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
> > > >
> > > > On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > > > > It is set by the driver:
> > > > >
> > > > > https://patchwork.freedesktop.org/patch/315291/
> > > > >
> > > > > (This doesn't really belong in devicetree, since it isn't a
> > > > > description of the hardware, so the driver is really the only place to
> > > > > set this.. which is fine because it is about a detail of how the
> > > > > driver works.)
> > > >
> > > > It is more a detail about how the firmware works. IIUC the problem is
> > > > that the firmware initializes the context mappings for the GPU and the
> > > > OS doesn't know anything about that and just overwrites them, causing
> > > > the firmware GPU driver to fail badly.
> > > >
> > > > So I think it is the task of the firmware to tell the OS not to touch
> > > > the devices mappings until the OS device driver takes over. On x86 there
> > > > is something similar with the RMRR/unity-map tables from the firmware.
> > > >
> > >
> > > Bjorn had a patchset[1] to inherit the config from firmware/bootloader
> > > when arm-smmu is probed which handles that part of the problem.  My
> > > patch is intended to be used on top of his patchset.  This seems to me
> > > like the best solution, if we don't have control over the firmware.
> >
> > Hmm, but the feedback from Robin on the thread you cite was that this should
> > be generalised to look more like RMRR, so there seems to be a clear message
> > here.
> >
> 
> Perhaps it is a lack of creativity, or lack of familiarity w/ iommu vs
> virtualization, but I'm not quite seeing how RMRR would help.. in
> particular when dealing with both DT and ACPI cases.

Well, I suppose we'd have something for DT and something for ACPI and we'd
try to make them look similar enough that we don't need lots of divergent
code in the kernel. The RMRR-like description would describe that, for a
particular device, a specific virt:phys mapping needs to exist in the
small window between initialising the SMMU and re-initialising the device
(GPU).

I would prefer this to be framebuffer-specific, since we're already in
flagrant violation of the arm64 boot requirements wrt ongoing DMA and making
this too general could lead to all sorts of brain damage. That would
probably also allow us to limit the flexibility, by mandating things like
alignment and memory attributes.

Having said that, I just realised I'm still a bit confused about the
problem: why does the bootloader require SMMU translation for the GPU at
all? If we could limit this whole thing to be identity-mapped/bypassed,
then all we need is a per-device flag in the firmware to indicate that the
SMMU should be initialised to allow passthrough for transactions originating
from that device.

> So I kinda prefer, when possible, if arm-smmu can figure out what is going
> on by looking at the hw state at boot (since that approach would work
> equally well for DT and ACPI).

That's not going to fly.

Forcing Linux to infer the state of the system by effectively parsing the
hardware configuration directly is fragile, error-prone and may not even be
possible in the general case. Worse, if this goes wrong, the symptoms are
very likely to be undiagnosable memory corruption, which is pretty awful in
my opinion.

Not only would you need separate parsing code for every IOMMU out there,
but you'd also need to make Linux aware of device aspects that it otherwise
doesn't care about, just in case the firmware decided to use them.
Furthermore, running an older kernel on newer hardware (which may have some
extensions), could cause the parsing to silently ignore parts of the device
that indicate memory regions which are in use. On top of that, there made be
device-global state that we are unable to reconfigure and that affect
devices other than the ones in question.

So no, I'm very much against this approach and the solution absolutely needs
to come in the form of a more abstract description from firmware.

> I *think* (but need to confirm if Bjorn hasn't already) that the
> memory for the pagetables that firmware/bootloader sets up is already
> removed from the memory map efi passes to kernel, so we don't need to
> worry about kernel stomping in-use pagetables.

It's precisely this sort of fragility that makes me nervous about this whole
approach.

Will

Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

[Rob Clark]

On Wed, Jul 24, 2019 at 3:51 AM Will Deacon <will@kernel.org> wrote:
>
> On Tue, Jul 23, 2019 at 10:40:55AM -0700, Rob Clark wrote:
> > On Tue, Jul 23, 2019 at 8:38 AM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Mon, Jul 22, 2019 at 09:23:48AM -0700, Rob Clark wrote:
> > > > On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
> > > > >
> > > > > On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > > > > > It is set by the driver:
> > > > > >
> > > > > > https://patchwork.freedesktop.org/patch/315291/
> > > > > >
> > > > > > (This doesn't really belong in devicetree, since it isn't a
> > > > > > description of the hardware, so the driver is really the only place to
> > > > > > set this.. which is fine because it is about a detail of how the
> > > > > > driver works.)
> > > > >
> > > > > It is more a detail about how the firmware works. IIUC the problem is
> > > > > that the firmware initializes the context mappings for the GPU and the
> > > > > OS doesn't know anything about that and just overwrites them, causing
> > > > > the firmware GPU driver to fail badly.
> > > > >
> > > > > So I think it is the task of the firmware to tell the OS not to touch
> > > > > the devices mappings until the OS device driver takes over. On x86 there
> > > > > is something similar with the RMRR/unity-map tables from the firmware.
> > > > >
> > > >
> > > > Bjorn had a patchset[1] to inherit the config from firmware/bootloader
> > > > when arm-smmu is probed which handles that part of the problem.  My
> > > > patch is intended to be used on top of his patchset.  This seems to me
> > > > like the best solution, if we don't have control over the firmware.
> > >
> > > Hmm, but the feedback from Robin on the thread you cite was that this should
> > > be generalised to look more like RMRR, so there seems to be a clear message
> > > here.
> > >
> >
> > Perhaps it is a lack of creativity, or lack of familiarity w/ iommu vs
> > virtualization, but I'm not quite seeing how RMRR would help.. in
> > particular when dealing with both DT and ACPI cases.
>
> Well, I suppose we'd have something for DT and something for ACPI and we'd
> try to make them look similar enough that we don't need lots of divergent
> code in the kernel. The RMRR-like description would describe that, for a
> particular device, a specific virt:phys mapping needs to exist in the
> small window between initialising the SMMU and re-initialising the device
> (GPU).

For both DT and ACPI (or perhaps more accurately UEFI and non-UEFI) we
often don't have much/any control of the firmware.  In the UEFI case,
we can at least get the physical address of the scanout buffer from
EFI GOP (since VA=PA at that point).  In either case we could get the
iova by reading back controller state.  We kinda just need the iommu
driver to not change anything about the context bank the display is
using until the display driver is ready to install it's own
pagetables.

Initially I just want to shut down display, and then bring it back up
w/ my own pagetables.. but eventually, once iommu/clk/genpd issues are
sorted upstream, I'm planning to read out more completely the display
state, and remap the existing scanout buffer at same iova in my own
pagetables/iommu_domain, for a flicker-free display handover... that
is a lot more work but at least it is self contained in the display
(and bridge/panel) drivers.

> I would prefer this to be framebuffer-specific, since we're already in
> flagrant violation of the arm64 boot requirements wrt ongoing DMA and making
> this too general could lead to all sorts of brain damage. That would
> probably also allow us to limit the flexibility, by mandating things like
> alignment and memory attributes.

I'd be pretty happy if we could convince qcom to use EFI GOP on
android devices too..

Although there is a lot more activity these days with people bringing
upstream support to various existing android phones/tablets.. and I'd
like to see that continue without downstream hacks due to lack of
control over firmware.

> Having said that, I just realised I'm still a bit confused about the
> problem: why does the bootloader require SMMU translation for the GPU at
> all? If we could limit this whole thing to be identity-mapped/bypassed,
> then all we need is a per-device flag in the firmware to indicate that the
> SMMU should be initialised to allow passthrough for transactions originating
> from that device.

I was chatting last night w/ Bjorn on IRC.. and he mentioned that it
looked like TTBRn was 0x0.  Which I wasn't expecting, and I didn't
realize was a legit thing.  Maybe the purpose is to allow display to
access memory w/ iova==pa but disallow memory access from other
devices using different context banks of the same iommu?  Maybe this
makes more sense to you?

> > So I kinda prefer, when possible, if arm-smmu can figure out what is going
> > on by looking at the hw state at boot (since that approach would work
> > equally well for DT and ACPI).
>
> That's not going to fly.
>
> Forcing Linux to infer the state of the system by effectively parsing the
> hardware configuration directly is fragile, error-prone and may not even be
> possible in the general case. Worse, if this goes wrong, the symptoms are
> very likely to be undiagnosable memory corruption, which is pretty awful in
> my opinion.

So, I guess in a DT world, we could have some boot-on; flag in the
device node.. that tells the iommu driver the stream-id(s) which might
be in use.  I guess the iommu driver would still need to map that back
to context bank, and then leave that CB and stream<->CB mapping alone.
But I think that would at least prevent "false positives", which I
think is what you are worrying about.

I'm not sure how this would work for ACPI, but tbh I haven't looked
yet at how iommu and device are connected w/ ACPI.

> Not only would you need separate parsing code for every IOMMU out there,
> but you'd also need to make Linux aware of device aspects that it otherwise
> doesn't care about, just in case the firmware decided to use them.
> Furthermore, running an older kernel on newer hardware (which may have some
> extensions), could cause the parsing to silently ignore parts of the device
> that indicate memory regions which are in use. On top of that, there made be
> device-global state that we are unable to reconfigure and that affect
> devices other than the ones in question.

I'm not sure you can avoid having *some* amount of code in each iommu
driver, since the exact meaning of iommu-cells can be different.. but
I guess we could come up with a generic way of determining which
device(s) attached to an iommu might be active when the iommu driver
probes.

> So no, I'm very much against this approach and the solution absolutely needs
> to come in the form of a more abstract description from firmware.
>
> > I *think* (but need to confirm if Bjorn hasn't already) that the
> > memory for the pagetables that firmware/bootloader sets up is already
> > removed from the memory map efi passes to kernel, so we don't need to
> > worry about kernel stomping in-use pagetables.
>
> It's precisely this sort of fragility that makes me nervous about this whole
> approach.

See above about TTBRn==null.. although if there were actually
pagetables, they would need to be reserved memory, otherwise I guess
windows would also have a problem w/ stomping pagetables.

BR,
-R