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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