Subject: Re: [PATCH] [v5] net: emac: emac gigabit ethernet controller driver

From: Timur Tabi <[email-masked]>
Date: Mon, 20 Jun 2016 13:15:27 -0500

Shanker Donthineni wrote:
>          /* Set dma_mask and coherent_dma_mask to 64-bits,
>           * if xHC supports 64-bit addressing */
>          if (HCC_64BIT_ADDR(xhci->hcc_params) &&
>                          !dma_set_mask(dev, DMA_BIT_MASK(64))) {
>                  xhci_dbg(xhci, "Enabling 64-bit DMA addresses.\n");
>                  dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
>          } else {
>                  /*
>                   * This is to avoid error in cases where a 32-bit USB
>                   * controller is used on a 64-bit capable system.
>                   */
>                  retval = dma_set_mask(dev, DMA_BIT_MASK(32));

I'm not sure this example is valid because HCC_64BIT_ADDR is part of the 
XCHI specification, so there's an architected way determine whether the 
platform is 64-bit capable or not.  The EMAC has nothing like that.

I can do this:

	ret = dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64));
	if (ret)
		dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(32))

but this has always seemed wrong to me, because it doesn't make sense to 
me that DMA_BIT_MASK(64) could ever fail.  DMA_BIT_MASK(64) says that 
the device can handle any physical address, so the device does not 
impose any limitations.  How could that fail?  I have has this question 
multiple times, and I have never gotten a satisfactory answer.

Also, I don't know if I should be using dma_set_mask_and_coherent or 
dma_coerce_mask_and_coherent.  The comment for 
dma_coerce_mask_and_coherent says this:

/*
  * Similar to the above, except it deals with the case where the device
  * does not have dev->dma_mask appropriately setup.
  */

How can I know if the device has dev->dma_mask "appropriately setup"?

Remember, I need a solution that works for DT and ACPI.

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