Re: [REGRESSION] Failed buffer allocation in Tegra fbdev

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Jan 23, 2024 at 02:33:15PM +0000, diogo.ivo@tecnico.ulisboa.pt wrote:
> Commit c8cc2655cc6c in the recent IOMMU changes breaks Tegra fbdev
> at least on the Pixel C with the following error message reporting
> a failed buffer allocation:
> 
> [    1.857660] drm drm: failed to allocate buffer of size 18432000
> 
> This error message is printed from tegra_bo_alloc() which is called by
> tegra_bo_create() in tegra_fbdev_probe(), which may indicate that other
> allocations would fail as well, not just the framebuffer.

Presumably this is because iommu_map_sgtable() under
tegra_bo_iommu_map() fails?

Which I suspect is because of the logic in
host1x_client_iommu_attach().

After c8cc2655cc6c iommu_get_domain_for_dev() will never return NULL.

So this:

	if (domain && domain != tegra->domain)
		return 0;

Will happen and the domain will be left at IDENTITY, while I suppose
the tegra_bo_iommu_map() assumes the domain was switched to tegra->domain.

Does this solve your issue?

diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
index ff36171c8fb700..15c7910b2e1c76 100644
--- a/drivers/gpu/drm/tegra/drm.c
+++ b/drivers/gpu/drm/tegra/drm.c
@@ -960,7 +960,7 @@ int host1x_client_iommu_attach(struct host1x_client *client)
         * not the shared IOMMU domain, don't try to attach it to a different
         * domain. This allows using the IOMMU-backed DMA API.
         */
-       if (domain && domain != tegra->domain)
+       if (domain && domain->type != IOMMU_DOMAIN_IDENTITY && domain != tegra->domain)
                return 0;
 
        if (tegra->domain) {

> This may be connected with an error in of_iommu_configure() that
> became visible after commit 6ff6e184f1f4d:
> 
> [    1.200004] host1x drm: iommu configuration for device failed with -ENOENT

Hmmm

This is a new logging, so it doesn't necessarily mean something has
changed in the code flow.

It seems the issue is something in there is returning ENOENT when it
probably should be ENODEV, but I haven't been able to guess where it
comes from.

Can you do some tracing and figure out where under
of_iommu_configure() this ENOENT return code is from?

Jason