ARM network performance and dma_mask (was: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s)

[Willy Tarreau <w@1wt.eu>]

Hi Rob,

While we were diagnosing a network performance regression that we finally
found and fixed, it appeared during a test that Linus' tree shows a much
higher performance on Armada 370 (armv7) than its predecessors. I can
saturate the two Gig links of my Mirabox each with a single TCP flow and
keep up to 25% of idle CPU in the optimal case. In 3.12.1 or 3.10.20, I
can achieve around 1.3 Gbps when the two ports are used in parallel.

Today I bisected these kernels to find what was causing this difference.
I found it was your patch below which I can copy entirely here :

  commit 0589342c27944e50ebd7a54f5215002b6598b748
  Author: Rob Herring <rob.herring@calxeda.com>
  Date:   Tue Oct 29 23:36:46 2013 -0500

      of: set dma_mask to point to coherent_dma_mask
    
      Platform devices created by DT code don't initialize dma_mask pointer to
      anything. Set it to coherent_dma_mask by default if the architecture
      code has not set it.
    
      Signed-off-by: Rob Herring <rob.herring@calxeda.com>

  diff --git a/drivers/of/platform.c b/drivers/of/platform.c
  index 9b439ac..c005495 100644
  --- a/drivers/of/platform.c
  +++ b/drivers/of/platform.c
  @@ -216,6 +216,8 @@ static struct platform_device *of_platform_device_create_pdata(
          dev->archdata.dma_mask = 0xffffffffUL;
   #endif
          dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
  +       if (!dev->dev.dma_mask)
  +               dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
          dev->dev.bus = &platform_bus_type;
          dev->dev.platform_data = platform_data;

And I can confirm that applying this patch on 3.10.20 + the fixes we found
yesterday substantially boosted my network performance (and reduced the CPU
usage when running on a single link).

I'm not at ease with these things so I'd like to ask your opinion here, is
this supposed to be an improvement or a fix ? Is this something we should
backport into stable versions, or is there something to fix in the armada
platform so that it works just as if the patch was applied ?

Thanks,
Willy