Re: [PATCH v2 2/2] Platform/RaspberryPi: Increase genet dma window

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 12 May 2021 at 00:37, Jeremy Linton <jeremy.linton@arm.com> wrote:
>
> The genet is capable of addressing the entire memory space
> on the RPI4. Lets allow it to dma into those regions.
> This solves intermittent issues with grub/etc being able
> to communicate when the 3G limit is lifted on 8G boards.
>
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> Reviewed-by: Jared McNeill <jmcneill@invisible.ca>
> ---
>  Platform/RaspberryPi/RPi4/RPi4.dsc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Platform/RaspberryPi/RPi4/RPi4.dsc b/Platform/RaspberryPi/RPi4/RPi4.dsc
> index 5c6783eae7..cf796acf6a 100644
> --- a/Platform/RaspberryPi/RPi4/RPi4.dsc
> +++ b/Platform/RaspberryPi/RPi4/RPi4.dsc
> @@ -717,7 +717,7 @@
>    Silicon/Broadcom/Drivers/Net/BcmGenetDxe/BcmGenetDxe.inf {
>      <PcdsFixedAtBuild>
>        gEmbeddedTokenSpaceGuid.PcdDmaDeviceOffset|0x00000000
> -      gEmbeddedTokenSpaceGuid.PcdDmaDeviceLimit|0xffffffff
> +      gEmbeddedTokenSpaceGuid.PcdDmaDeviceLimit|0xffffffffff
>    }
>
>    #
> --
> 2.13.7
>

As I said before, I am fine with this patch, but I would still like to
understand why this causes a failure. The device limit is used by the
DMA layer to decide whether bounce buffering is needed or not, but the
API should work as expected either way, with perhaps only an impact on
performance if bounce buffering is done unnecessarily.

What kind of errors are you seeing (and fixing) with this change?