linux-kernel - Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT

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Date:   Thu, 28 Sep 2023 16:47:07 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jim Quinlan <james.quinlan@...adcom.com>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     Christoph Hellwig <hch@....de>,
        bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Eric DeVolder <eric.devolder@...cle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        Claire Chang <tientzu@...omium.org>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

On 28/09/2023 1:07 pm, Jim Quinlan wrote:
> On Wed, Sep 27, 2023 at 7:10 PM Linus Walleij <linus.walleij@...aro.org> wrote:
>>
>> Hi Jim,
>>
>> thanks for your patch!
>>
>> On Tue, Sep 26, 2023 at 7:52 PM Jim Quinlan <james.quinlan@...adcom.com> wrote:
>>
>>> Without this commit, the use of dma_alloc_coherent() while
>>> using CONFIG_DMA_RESTRICTED_POOL=y breaks devices from working.
>>> For example, the common Wifi 7260 chip (iwlwifi) works fine
>>> on arm64 with restricted memory but not on arm, unless this
>>> commit is applied.
>>>
>>> Signed-off-by: Jim Quinlan <james.quinlan@...adcom.com>
>>
>> (...)
>>> +       select DMA_DIRECT_REMAP
>>
>> Christoph invented that symbol so he can certainly
>> explain what is missing to use this on ARM.
>>
>> This looks weird to me, because:
>>> git grep atomic_pool_init
>> arch/arm/mm/dma-mapping.c:static int __init atomic_pool_init(void)
>> kernel/dma/pool.c:static int __init dma_atomic_pool_init(void)
>>
>> Now you have two atomic DMA pools in the kernel,
>> and a lot more than that is duplicated. I'm amazed that it
>> compiles at all.
>>
>> Clearly if you want to do this, surely the ARM-specific
>> arch/arm/mm/dma-mapping.c and arch/arm/mm/dma-mapping-nommu.c
>> needs to be removed at the same time?
>>
>> However I don't think it's that simple, because Christoph would surely
>> had done this a long time ago if it was that simple.
> 
> Hello Linus,
> 
> Yes, this is the reason I used "RFC" as the fix looked too easy to be viable :-)
> I debugged it enough to see that the host driver's
> writes to the dma_alloc_coherent() region  were not appearing in
> memory, and that
> led me to DMA_DIRECT_REMAP.

Oh, another thing - the restricted-dma-pool is really only for streaming 
DMA - IIRC there can be cases where the emergency fallback of trying to 
allocate out of the bounce buffer won't work properly. Are you also 
using an additional shared-dma-pool carveout to satisfy the coherent 
allocations, per the DT binding?

Thanks,
Robin.