linux-kernel - Re: [PATCH 02/21] xtensa: dma-mapping: use normal cache invalidation rules

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Date:   Mon, 27 Mar 2023 08:42:59 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Vineet Gupta <vgupta@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Stafford Horne <shorne@...il.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
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        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH 02/21] xtensa: dma-mapping: use normal cache invalidation rules

On Mon, Mar 27, 2023 at 5:14 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> xtensa is one of the platforms that has both write-back and write-through
> caches, and needs to account for both in its DMA mapping operations.
>
> It does this through a set of operations that is different from any
> architecture. This is not a problem by itself, but it makes it rather
> hard to figure out whether this is correct or not, and to unify this
> implementation with the others.
>
> Change the semantics to the usual ones for non-speculating CPUs:
>
>  - On DMA_TO_DEVICE, call __flush_dcache_range() to perform the
>    writeback even on writethrough caches, where this is a nop.
>
>  - On DMA_FROM_DEVICE, invalidate the mapping before the DMA rather
>    than afterwards.
>
>  - On DMA_BIDIRECTIONAL, combine the pre-writeback with the
>    post-invalidate into a call to __flush_invalidate_dcache_range()
>    that turns into a simple invalidate on writeback caches.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/xtensa/Kconfig                  |  1 -
>  arch/xtensa/include/asm/cacheflush.h |  6 +++---
>  arch/xtensa/kernel/pci-dma.c         | 29 +++++-----------------------
>  3 files changed, 8 insertions(+), 28 deletions(-)

Reviewed-by: Max Filippov <jcmvbkbc@...il.com>

-- 
Thanks.
-- Max