Re: [PATCH net 1/3] net: mvpp2: fix the dma_mask and coherent_dma_mask settings for PPv2.2

From: Antoine Tenart <redacted>
Date: Mon, 18 Sep 2017 15:04:06 +0200

The dev->dma_mask usually points to dev->coherent_dma_mask. This is an
issue as setting both of them will override the other. This is
problematic here as the PPv2 driver uses a 32-bit-mask for coherent
accesses (txq, rxq, bm) and a 40-bit mask for all other accesses due to
an hardware limitation.

This can lead to a memory remap for all dma_map_single() calls when
dealing with memory above 4GB.

Fixes: 2067e0a13cfe ("net: mvpp2: set dma mask and coherent dma mask on PPv2.2")
Reported-by: Stefan Chulski <redacted>
Signed-off-by: Antoine Tenart <redacted>

Yikes.

I surrmise that if the platform has made dev->dma_mask point to
&dev->coherent_dma_mask, it is because it does not allow the two
settings to be set separately.

By rearranging the pointer, you are bypassing that, and probably
breaking things or creating a situation that the DMA mapping
layer is not expecting.

I want to know more about the situations where dma_mask is set to
point to &coherent_dma_mask and how that is supposed to work.

At a minimum this commit log message needs to go into more detail.

Thanks.