Arnd Bergmann wrote:
This is basically ok, but then I think you should pass GFP_DMA
or GFP_DMA32 to all allocations that the driver does after
the 64-bit mask fails, otherwise you get a significant overhead
in the bounce buffers.
Well, for starters, ZONE_DMA32 is the same as ZONE_NORMAL on ARM,
because CONFIG_ZONE_DMA32 is not defined.
#ifdef CONFIG_ZONE_DMA32
#define OPT_ZONE_DMA32 ZONE_DMA32
#else
#define OPT_ZONE_DMA32 ZONE_NORMAL
#endif
(I wonder if this should say instead:
#ifdef CONFIG_ZONE_DMA32
#define OPT_ZONE_DMA32 ZONE_DMA32
#else
#define OPT_ZONE_DMA32 ZONE_DMA <----
#endif
)
However, I'm not sure where I should be using GFP_DMA anyway. Whenever
the driver allocates memory for DMA, it uses dma_zalloc_coherent():
ring_header->v_addr = dma_zalloc_coherent(dev, ring_header->size,
&ring_header->dma_addr,
GFP_KERNEL);
and I don't think I need to pass GFP_DMA to dma_zalloc_coherent. Every
other memory allocation is a kmalloc variant, but that's never for DMA,
so that memory can be anywhere.
I found about 70 drivers that fall-back to 32-bit DMA if 64-bit fails.
None of them do as you suggest. They all just set the mask to 64 or 32
and that's it.
Some drivers set NETIF_F_HIGHDMA if 64-bit DMA is enabled:
if (pci_using_dac)
netdev->features |= NETIF_F_HIGHDMA;
I could do this, but I think it has no meaning on ARM64 because it
depends on CONFIG_HIGHMEM.
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