Re: [PATCH] ArmPlatformPkg: remove EFI_MEMORY_UC attribute from normal memory

["Cohen, Eugene" <eugene@hp.com>]

Ard,

> So remove the EFI_MEMORY_UC attribute that we set by default on
> system RAM.
> If any region requires this attribute, it is up to the driver to set this
> attribute, and to ensure that no offending operations are performed
> on it.
>

For DMA common-buffer operations on systems without snooping DMA capabilities, UC or WC mapping of system memory regions is required.

>        EFI_RESOURCE_ATTRIBUTE_PRESENT |
>        EFI_RESOURCE_ATTRIBUTE_INITIALIZED |
> -      EFI_RESOURCE_ATTRIBUTE_UNCACHEABLE |
>        EFI_RESOURCE_ATTRIBUTE_WRITE_COMBINEABLE |
>        EFI_RESOURCE_ATTRIBUTE_WRITE_THROUGH_CACHEABLE |
>        EFI_RESOURCE_ATTRIBUTE_WRITE_BACK_CACHEABLE |

The EFI_RESOURCE_ATTRIBUTE_UNCACHEABLE bit your removing is removing the capability for uncacheable memory such that even if a driver wanted to make a DMA buffer uncacheable GCD will no longer allow this because the resource does not support this capability.

Is it your intent to indicate that system memory is no longer capable of being uncacheable?  If so how would you plan to accomodate the DMA use case for GCD SetMemorySpaceAttributes?

Thanks,

Eugene