Re: [PATCH v5 04/14] driver core: platform: Add driver dma ownership management

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jan 04, 2022 at 09:56:34AM +0800, Lu Baolu wrote:
> Multiple platform devices may be placed in the same IOMMU group because
> they cannot be isolated from each other. These devices must either be
> entirely under kernel control or userspace control, never a mixture. This
> checks and sets DMA ownership during driver binding, and release the
> ownership during driver unbinding.
> 
> The device driver may set a new flag (no_kernel_api_dma) to skip calling
> iommu_device_use_dma_api() during the binding process. For instance, the
> userspace framework drivers (vfio etc.) which need to manually claim
> their own dma ownership when assigning the device to userspace.
> 
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
>  include/linux/platform_device.h |  1 +
>  drivers/base/platform.c         | 20 ++++++++++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
> index 17fde717df68..8f0eaafcef47 100644
> --- a/include/linux/platform_device.h
> +++ b/include/linux/platform_device.h
> @@ -210,6 +210,7 @@ struct platform_driver {
>  	struct device_driver driver;
>  	const struct platform_device_id *id_table;
>  	bool prevent_deferred_probe;
> +	bool no_kernel_api_dma;
>  };
>  
>  #define to_platform_driver(drv)	(container_of((drv), struct platform_driver, \
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index b4d36b46ab2e..d5171e44d903 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -30,6 +30,7 @@
>  #include <linux/property.h>
>  #include <linux/kmemleak.h>
>  #include <linux/types.h>
> +#include <linux/iommu.h>
>  
>  #include "base.h"
>  #include "power/power.h"
> @@ -1451,9 +1452,16 @@ static void platform_shutdown(struct device *_dev)
>  
>  static int platform_dma_configure(struct device *dev)
>  {
> +	struct platform_driver *drv = to_platform_driver(dev->driver);
>  	enum dev_dma_attr attr;
>  	int ret = 0;
>  
> +	if (!drv->no_kernel_api_dma) {
> +		ret = iommu_device_use_dma_api(dev);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	if (dev->of_node) {
>  		ret = of_dma_configure(dev, dev->of_node, true);
>  	} else if (has_acpi_companion(dev)) {
> @@ -1461,9 +1469,20 @@ static int platform_dma_configure(struct device *dev)
>  		ret = acpi_dma_configure(dev, attr);
>  	}
>  
> +	if (ret && !drv->no_kernel_api_dma)
> +		iommu_device_unuse_dma_api(dev);

So you are now going to call this for every platform driver _unless_
they set this flag?

And having "negative" flags is rough to parse at times.  Yes, we have
"prevent_deferred_probe" already here, so maybe keep this in the same
nameing scheme and use "prevent_dma_api"?

And it would be good to document this somewhere as to what this means.

thanks,

greg k-h