Re: [PATCH 1/2] drm/xe/dgfx: Block rpm for active mmap mappings

[Matthew Auld <matthew.auld@intel.com>]

On 06/12/2023 13:34, Badal Nilawar wrote:
> Block rpm for discrete cards when mmap mappings are active.
> Ideally rpm wake ref should be taken in vm_open call and put in vm_close
> call but it is seen that vm_open doesn't get called for xe_gem_vm_ops.
> Therefore rpm wake ref is being get in xe_drm_gem_ttm_mmap and put
> in vm_close.
> 
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Anshuman Gupta <anshuman.gupta@intel.com>
> Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
> ---
>   drivers/gpu/drm/xe/xe_bo.c | 35 +++++++++++++++++++++++++++++++++--
>   1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 72dc4a4eed4e..5741948a2a51 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -15,6 +15,7 @@
>   #include <drm/ttm/ttm_tt.h>
>   #include <drm/xe_drm.h>
>   
> +#include "i915_drv.h"

Do we need this?

>   #include "xe_device.h"
>   #include "xe_dma_buf.h"
>   #include "xe_drm_client.h"
> @@ -1158,17 +1159,47 @@ static vm_fault_t xe_gem_fault(struct vm_fault *vmf)
>   	return ret;
>   }
>   
> +static void xe_ttm_bo_vm_close(struct vm_area_struct *vma)
> +{
> +	struct ttm_buffer_object *tbo = vma->vm_private_data;
> +	struct drm_device *ddev = tbo->base.dev;
> +	struct xe_device *xe = to_xe_device(ddev);
> +
> +	ttm_bo_vm_close(vma);
> +
> +	if (tbo->resource->bus.is_iomem)
> +		xe_device_mem_access_put(xe);
> +}
> +
>   static const struct vm_operations_struct xe_gem_vm_ops = {
>   	.fault = xe_gem_fault,
>   	.open = ttm_bo_vm_open,
> -	.close = ttm_bo_vm_close,
> +	.close = xe_ttm_bo_vm_close,
>   	.access = ttm_bo_vm_access
>   };
>   
> +int xe_drm_gem_ttm_mmap(struct drm_gem_object *gem,
> +			struct vm_area_struct *vma)
> +{
> +	struct ttm_buffer_object *tbo = drm_gem_ttm_of_gem(gem);
> +	struct drm_device *ddev = tbo->base.dev;
> +	struct xe_device *xe = to_xe_device(ddev);
> +	int ret;
> +
> +	ret = drm_gem_ttm_mmap(gem, vma);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (tbo->resource->bus.is_iomem)
> +		xe_device_mem_access_get(xe);

Checking is_iomem outside of the usual locking is racy. One issue here 
is that is_iomem can freely change at any point (like at fault time) so 
when vm_close is called you can easily get an an unbalanced RPM ref 
count. For example io_mem is false here but later becomes true in 
bo_vm_close and then we call mem_access_put even though we never called 
mem_access_get.

Maybe check the possible placements of the object instead since that is 
immutable?

> +
> +	return 0;
> +}
> +
>   static const struct drm_gem_object_funcs xe_gem_object_funcs = {
>   	.free = xe_gem_object_free,
>   	.close = xe_gem_object_close,
> -	.mmap = drm_gem_ttm_mmap,
> +	.mmap = xe_drm_gem_ttm_mmap,
>   	.export = xe_gem_prime_export,
>   	.vm_ops = &xe_gem_vm_ops,
>   };