Jonathan Corbet <corbet@xxxxxxx> writes: > On Mon, 17 Oct 2016 16:26:23 +0100 > Punit Agrawal <punit.agrawal@xxxxxxx> wrote: > >> The dma mapping api howto gives the impression that using the >> dma_set_mask_and_coherent (and related DMA APIs) will cause the kernel >> to check all the components in the path from the device to memory for >> addressing restrictions. In systems with address translations between >> the device and memory (e.g., when using IOMMU), this implies that a >> successful call to set set dma mask has checked the addressing >> constraints of the intermediaries as well. >> >> For the IOMMU drivers in the tree, the check is actually performed while >> allocating the DMA buffer rather than when the DMA mask is >> configured. For MMUs that do not support the full device addressing >> capability, the allocations are made from a reduced address space. >> >> Update the documentation to clarify that even though the call to >> dma_set_mask_and_coherent succeeds, it may not be possible to use the >> full addressing capability of the device. > > OK, so I guess I can buy this. But... > >> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx> >> Cc: Jonathan Corbet <corbet@xxxxxxx> >> --- >> Documentation/DMA-API-HOWTO.txt | 39 +++++++++++++++++++++++---------------- >> 1 file changed, 23 insertions(+), 16 deletions(-) >> >> diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt >> index 979228b..240d1ee 100644 >> --- a/Documentation/DMA-API-HOWTO.txt >> +++ b/Documentation/DMA-API-HOWTO.txt >> @@ -159,39 +159,46 @@ support 64-bit addressing (DAC) for all transactions. And at least >> one platform (SGI SN2) requires 64-bit consistent allocations to >> operate correctly when the IO bus is in PCI-X mode. >> >> -For correct operation, you must interrogate the kernel in your device >> -probe routine to see if the DMA controller on the machine can properly >> -support the DMA addressing limitation your device has. It is good >> +For correct operation, you must inform the kernel in your device probe >> +routine to see if the DMA controller on the machine can properly >> +support the DMA addressing capabilities your device has. It is good > > Here it's still saying "to see if the DMA controller on the machine can > properly support the DMA addressing capabilities your device has". So > you've not really changed the sense of this sentence here. You're right - the changes don't go far enough. How about dropping the bit so that it now reads - "For correct operation, in your device probe routine, you must inform the DMA addressing capabilities of your device to the kernel." > > If I understand things correctly, the calls in question are storing the > device's limitations; they will only fail if the kernel is entirely > unable to work within the indicated range, right? I don't think there's > ever been any guarantee that the system as a whole could use the entire > range that is addressable by the device. That matches my understanding as well. I was just trying to make it more explicit with this patch. > I have no objection to making > that more clear, but let's actually make it more clear by saying what the > functions are actually doing. > > Make sense, or am I missing something here? Let me know if the change I suggest is an improvement and I'll send out a v2. Thanks, Punit > > Thanks, > > jon -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
