linux-kernel - Re: [PATCH 3/4] riscv: Implement Zicbom-based cache management operations

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Date:   Fri, 24 Jun 2022 09:49:09 +0200
From:   Heiko Stuebner <heiko@...ech.de>
To:     Christoph Hellwig <hch@....de>
Cc:     palmer@...belt.com, paul.walmsley@...ive.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        wefu@...hat.com, guoren@...nel.org, cmuellner@...ux.com,
        philipp.tomsich@...ll.eu, hch@....de, samuel@...lland.org,
        atishp@...shpatra.org, anup@...infault.org, mick@....forth.gr,
        robh+dt@...nel.org, krzk+dt@...nel.org, devicetree@...r.kernel.org,
        drew@...gleboard.org, rdunlap@...radead.org,
        Atish Patra <atish.patra@....com>
Subject: Re: [PATCH 3/4] riscv: Implement Zicbom-based cache management operations

Hi Christoph,

Am Montag, 20. Juni 2022, 08:16:07 CEST schrieb Christoph Hellwig:
> On Sun, Jun 19, 2022 at 10:32:11PM +0200, Heiko Stuebner wrote:
> > +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> > +#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> > +#endif
> 
> This needs to be greater or equal to riscv_cbom_block_size, but the
> core code requires a compile time constant here.  So we'll need a big
> fat comment here, and panic if riscv_cbom_block_size is >
> L1_CACHE_BYTES/ARCH_DMA_MINALIGN in the code that queries
> riscv_cbom_block_size.

ARM people also had this nice WARN_TAINT to warn when the similar
case happens on ARM64 and the ARCH_DMA_MINALIGN is smaller than
the register value so I've added a similar mechanism.

I've read numerous mails from Torvalds over time that panic-ing should
only ever be the very very last resort, so that WARN_TAINT looks like
a less drastic option while still generating that big warning to users.


> Note that the arm64 folks are looking into making this variable or
> killing it off in this current form, so things might be getting better
> soon.
> 
> > +void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
> > +			      enum dma_data_direction dir)
> > +{
> > +	void *vaddr = phys_to_virt(paddr);
> > +
> > +	switch (dir) {
> > +	case DMA_TO_DEVICE:
> > +		ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
> > +		break;
> > +	case DMA_FROM_DEVICE:
> > +		ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
> > +		break;
> 
> For this also see:
> 
> https://lore.kernel.org/all/20220606152150.GA31568@willie-the-truck/
> 
> and
> 
> https://lore.kernel.org/linux-arm-kernel/20220610151228.4562-1-will@kernel.org/T/

so from that discussion, it looks like a "clean" should happen here to
prevent stale bytes (not written to by the dma transfer itself) in the
buffer area I guess.

I'll give that a spin :-)

> > +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> > +		const struct iommu_ops *iommu, bool coherent)
> > +{
> > +	dev->dma_coherent = coherent;
> > +}
> 
> This probably wants a sanity check warn if coherent if false without
> any support for cache flushing as that will cause data corruption.

I've added a riscv_noncoherent_supported() call that will track that
"somebody" implemented non-coherence functionality from their
setup function (zicbom_probe, thead_errata-probe) and a matching
second WARN_TAINT in arch_setup_dma_ops() when coherent value
and availability of non-coherence handling is not matched.

Heiko