Re: [Freedreno] [PATCH] drm/msm: Check for the GPU IOMMU during bind
From: Dmitry Baryshkov
Date: Fri Jul 07 2023 - 13:27:28 EST
On 07/07/2023 18:03, Jordan Crouse wrote:
On Thu, Jul 06, 2023 at 09:55:13PM +0300, Dmitry Baryshkov wrote:
On 10/03/2023 00:20, Jordan Crouse wrote:
While booting with amd,imageon on a headless target the GPU probe was
failing with -ENOSPC in get_pages() from msm_gem.c.
Investigation showed that the driver was using the default 16MB VRAM
carveout because msm_use_mmu() was returning false since headless devices
use a dummy parent device. Avoid this by extending the existing is_a2xx
priv member to check the GPU IOMMU state on all platforms and use that
check in msm_use_mmu().
This works for memory allocations but it doesn't prevent the VRAM carveout
from being created because that happens before we have a chance to check
the GPU IOMMU state in adreno_bind.
There are a number of possible options to resolve this but none of them are
very clean. The easiest way is to likely specify vram=0 as module parameter
on headless devices so that the memory doesn't get wasted.
This patch was on my plate for quite a while, please excuse me for
taking it so long.
No worries. I'm also chasing a bunch of other stuff too.
I see the following problem with the current code. We have two different
instances than can access memory: MDP/DPU and GPU. And each of them can
either have or miss the MMU.
For some time I toyed with the idea of determining whether the allocated
BO is going to be used by display or by GPU, but then I abandoned it. We
can have display BOs being filled by GPU, so handling it this way would
complicate things a lot.
This actually rings a tiny bell in my head with the idea of splitting
the display and GPU parts to two different drivers, but I'm not sure
what would be the overall impact.
As I now exclusively work on headless devices I would be 100% for this,
but I'm sure that our laptop friends might not agree :)
I do not know here. This is probably a question to Rob, as he better
understands the interaction between GPU and display parts of the userspace.
More on the msm_use_mmu() below.
Signed-off-by: Jordan Crouse <jorcrous@xxxxxxxxxx>
---
drivers/gpu/drm/msm/adreno/adreno_device.c | 6 +++++-
drivers/gpu/drm/msm/msm_drv.c | 7 +++----
drivers/gpu/drm/msm/msm_drv.h | 2 +-
3 files changed, 9 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
index 36f062c7582f..4f19da28f80f 100644
--- a/drivers/gpu/drm/msm/adreno/adreno_device.c
+++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
@@ -539,7 +539,11 @@ static int adreno_bind(struct device *dev, struct device *master, void *data)
DBG("Found GPU: %u.%u.%u.%u", config.rev.core, config.rev.major,
config.rev.minor, config.rev.patchid);
- priv->is_a2xx = config.rev.core == 2;
+ /*
+ * A2xx has a built in IOMMU and all other IOMMU enabled targets will
+ * have an ARM IOMMU attached
+ */
+ priv->has_gpu_iommu = config.rev.core == 2 || device_iommu_mapped(dev);
priv->has_cached_coherent = config.rev.core >= 6;
gpu = info->init(drm);
diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
index aca48c868c14..a125a351ec90 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -318,11 +318,10 @@ bool msm_use_mmu(struct drm_device *dev)
struct msm_drm_private *priv = dev->dev_private;
/*
- * a2xx comes with its own MMU
- * On other platforms IOMMU can be declared specified either for the
- * MDP/DPU device or for its parent, MDSS device.
+ * Return true if the GPU or the MDP/DPU or parent MDSS device has an
+ * IOMMU
*/
- return priv->is_a2xx ||
+ return priv->has_gpu_iommu ||
device_iommu_mapped(dev->dev) ||
device_iommu_mapped(dev->dev->parent);
I have a generic feeling that both old an new code is not fully correct.
Please correct me if I'm wrong:
We should be using VRAM, if either of the blocks can not use remapped
memory. So this should have been:
bool msm_use_mmu()
{
if (!gpu_has_iommu)
return false;
if (have_display_part && !display_has_mmu())
return false;
return true;
}
What do you think.
I would have to see (and try) the real code but that seems like it might
be reasonable.
Sure, let me craft it then.
I would like to hear from some of the a2xx users too
because this mostly affects them. On 3xx and newer you've always had the
option of not having a MMU for GPU or display but I can't think of any
use cases where you wouldn't want it.
msm8974 doesn't have (working) IOMMU driver. I also think there was an
issue somewhere around sdm630/660. And the wonderful msm8992/4, IIRC,
also don't have one.
Also the headless mode was quite useful for bringing up platforms, as it
allowed us to test GPU separately (and ofc. in some cases even w/o MMU).
I have both a2xx (only iMX for now) and a3xx for the tests here, on my
desk.
--
With best wishes
Dmitry