linux-kernel - Re: [PATCH kernel v4 2/2] powerpc/dma: Fallback to dma

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Message-ID: <878sbpwe30.fsf@mpe.ellerman.id.au>
Date:   Thu, 29 Oct 2020 20:56:03 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Alexey Kardashevskiy <aik@...abs.ru>, linuxppc-dev@...ts.ozlabs.org
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH kernel v4 2/2] powerpc/dma: Fallback to dma_ops when persistent memory present

Alexey Kardashevskiy <aik@...abs.ru> writes:
> So far we have been using huge DMA windows to map all the RAM available.
> The RAM is normally mapped to the VM address space contiguously, and
> there is always a reasonable upper limit for possible future hot plugged
> RAM which makes it easy to map all RAM via IOMMU.
>
> Now there is persistent memory ("ibm,pmemory" in the FDT) which (unlike
> normal RAM) can map anywhere in the VM space beyond the maximum RAM size
> and since it can be used for DMA, it requires extending the huge window
> up to MAX_PHYSMEM_BITS which requires hypervisor support for:
> 1. huge TCE tables;
> 2. multilevel TCE tables;
> 3. huge IOMMU pages.
>
> Certain hypervisors cannot do either so the only option left is
> restricting the huge DMA window to include only RAM and fallback to
> the default DMA window for persistent memory.
>
> This defines arch_dma_map_direct/etc to allow generic DMA code perform
> additional checks on whether direct DMA is still possible.
>
> This checks if the system has persistent memory. If it does not,
> the DMA bypass mode is selected, i.e.
> * dev->bus_dma_limit = 0
> * dev->dma_ops_bypass = true <- this avoid calling dma_ops for mapping.
>
> If there is such memory, this creates identity mapping only for RAM and
> sets the dev->bus_dma_limit to let the generic code decide whether to
> call into the direct DMA or the indirect DMA ops.
>
> This should not change the existing behaviour when no persistent memory
> as dev->dma_ops_bypass is expected to be set.
>
> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>

Acked-by: Michael Ellerman <mpe@...erman.id.au>

cheers