linux-kernel - Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent

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Message-ID: <CAPv3WKdcQGqofEgV4w_iiQ7FFa0ZF=du8gK9eAD==10HhwEnUA@mail.gmail.com>
Date:   Sun, 23 Oct 2022 23:30:34 +0200
From:   Marcin Wojtas <mw@...ihalf.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        gregory.clement@...tlin.com, sebastian.hesselbarth@...il.com,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org, hch@....de,
        kabel@...nel.org, jaz@...ihalf.com
Subject: Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent

niedz., 23 paź 2022 o 18:21 Russell King (Oracle)
<linux@...linux.org.uk> napisał(a):
>
> On Sun, Oct 23, 2022 at 05:04:01PM +0200, Andrew Lunn wrote:
> > On Sun, Oct 23, 2022 at 01:40:24AM +0200, Marcin Wojtas wrote:
> > > Armada 38x platforms marks all devices as coherent via
> > > mvebu_hwcc_notifier(), whereas the standard way to determine
> > > this is by of_dma_is_coherent(). Reflect the hardware
> > > capabilities by adding 'dma-coherent' properties to the device tree.
> >
> > Hi Marcin
> >
> > Does this need to go to -rc for 6.0? The DMA issues being reported?
> > If so, please add a Fixed: tag.
>
> Are we absolutely sure this makes sense?
>
> Looking at atch/arm/mach-mvebu/coherency.c, there are dependencies
> on stuff such as whether the kernel is in SMP mode or not (because
> the page tables need to be appropriately marked as shared for
> coherency with IO to work). We only enable the shared bit if we're
> in SMP mode because (a) its difficult to do at runtime due to TLB
> conflicts (requires switching the MMU off, rewriting the page tables
> and switching the MMU back on), and (b) setting the shared bit for
> CPUs that don't need it _can_ result in the CPUs basically bypassing
> their caches and thus kill system performance.
>
> So, if we have Armada 38x platforms that are operated in uniprocessor
> mode, this patch can cause havoc on such a setup.
>
> I would suggest utmost caution with this approach.
>

Sure. In such a case the description of 380 variant (single core)
should remain untouched.

We need to decide what to do with dual-CPU, i.e. Armada 385/388. How about:
- Don't change current behavior, i.e. perform a necessary kernel
configuration in "arm,pl310-cache" driver,
arch/arm/mach-mvebu/coherency.c + &coherencyfab:node in DT
- Satisfy of_dma_is_coherent() by adding `dma-coherent;` in
armada-385.dtsi only (IMO this would describe HW properly)
?

Best regards,
Marcin