linux-kernel - Re: [RFC 3/5] dma-mapping: Enable global non-coherent pool support for RISC-V

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Message-ID: <20210726070030.GB9035@lst.de>
Date:   Mon, 26 Jul 2021 09:00:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@....de>, devicetree@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Frank Rowand <frowand.list@...il.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        iommu@...ts.linux-foundation.org, linux-riscv@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: [RFC 3/5] dma-mapping: Enable global non-coherent pool support
 for RISC-V

On Fri, Jul 23, 2021 at 02:40:29PM -0700, Atish Patra wrote:
> Currently, linux,dma-default is used to reserve a global non-coherent pool
> to allocate memory for dma operations. This can be useful for RISC-V as
> well as the ISA specification doesn't specify a method to modify PMA
> attributes or page table entries to define non-cacheable area yet.
> A non-cacheable memory window is an alternate options for vendors to
> support non-coherent devices.

Please explain why you do not want to use the simply non-cachable
window support using arch_dma_set_uncached as used by mips, niops2 and
xtensa.

> +static int __dma_init_global_coherent(phys_addr_t phys_addr, dma_addr_t device_addr, size_t size)




>  {
>  	struct dma_coherent_mem *mem;
>  
> -	mem = dma_init_coherent_memory(phys_addr, phys_addr, size, true);
> +	if (phys_addr == device_addr)
> +		mem = dma_init_coherent_memory(phys_addr, device_addr, size, true);
> +	else
> +		mem = dma_init_coherent_memory(phys_addr, device_addr, size, false);

Nak.  The phys_addr != device_addr support is goign away.  This needs
to be filled in using dma-ranges property hanging of the struct device.