Re: [PATCH v1 1/3] gpu: host1x: Remove implicit IOMMU backing on client's registration

[Thierry Reding <thierry.reding@gmail.com>]

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On Sun, Jun 23, 2019 at 08:37:41PM +0300, Dmitry Osipenko wrote:
> On ARM32 we don't want any of the clients device to be backed by the
> implicit domain, simply because we can't afford such a waste on older
> Tegra SoCs that have very few domains available in total. The recent IOMMU
> support addition for the Video Decoder hardware uncovered the problem
> that an unfortunate drivers probe order results in the DRM driver probe
> failure if CONFIG_ARM_DMA_USE_IOMMU=y due to a shortage of IOMMU domains
> caused by the implicit backing. The host1x_client_register() is a common
> function that is invoked by all of the relevant DRM drivers during theirs
> probe and hence it is convenient to remove the implicit backing there,
> resolving the problem.
> 
> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> ---
>  drivers/gpu/host1x/bus.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)

I don't really want to do this in a central place like this. If we
really do need this, why can't we do it in the individual drivers? Also,
we already call host1x_client_iommu_attach() from all the drivers and
that detaches from the IOMMU as well. So I'm not sure I understand why
this is needed.

Thierry

> 
> diff --git a/drivers/gpu/host1x/bus.c b/drivers/gpu/host1x/bus.c
> index 742aa9ff21b8..559df3974afb 100644
> --- a/drivers/gpu/host1x/bus.c
> +++ b/drivers/gpu/host1x/bus.c
> @@ -14,6 +14,10 @@
>  #include "bus.h"
>  #include "dev.h"
>  
> +#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
> +#include <asm/dma-iommu.h>
> +#endif
> +
>  static DEFINE_MUTEX(clients_lock);
>  static LIST_HEAD(clients);
>  
> @@ -710,6 +714,21 @@ int host1x_client_register(struct host1x_client *client)
>  	struct host1x *host1x;
>  	int err;
>  
> +#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
> +	/*
> +	 * The client's driver could be backed by implicit IOMMU mapping
> +	 * and we don't want to have that because all of current Tegra
> +	 * drivers are managing IOMMU by themselves. This is a convenient
> +	 * place for unmapping of the implicit mapping because this function
> +	 * is called by all host1x drivers during theirs probe.
> +	 */
> +	if (client->dev->archdata.mapping) {
> +		struct dma_iommu_mapping *mapping =
> +			to_dma_iommu_mapping(client->dev);
> +		arm_iommu_detach_device(client->dev);
> +		arm_iommu_release_mapping(mapping);
> +	}
> +#endif
>  	mutex_lock(&devices_lock);
>  
>  	list_for_each_entry(host1x, &devices, list) {
> -- 
> 2.22.0
> 

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