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Subject: Re: [PATCH v4 1/2] tty: hvc: pass DMA capable memory to put_chars()
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     gregkh <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>, Amit Shah <amit@kernel.org>,
        Omar Sandoval <osandov@fb.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guo Ren <guoren@kernel.org>
References: <20210806030138.123479-1-xianting.tian@linux.alibaba.com>
 <20210806030138.123479-2-xianting.tian@linux.alibaba.com>
 <CAK8P3a2=BmVv0tvUKaca+LYxuAussAJtAJW9O3fRN2CbV2-9aw@mail.gmail.com>
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 <CAK8P3a2ykLvJkhX+wDAOHdyLHjPFAfhOxi5BNM9kTKv_8F7VQg@mail.gmail.com>
From:   Xianting TIan <xianting.tian@linux.alibaba.com>
Message-ID: <a5bb4fbb-a9c4-9b9a-3cfe-09d3b99c7915@linux.alibaba.com>
Date:   Thu, 12 Aug 2021 17:14:09 +0800
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在 2021/8/12 下午4:54, Arnd Bergmann 写道:
> On Thu, Aug 12, 2021 at 10:08 AM Xianting TIan
> <xianting.tian@linux.alibaba.com> wrote:
>> 在 2021/8/6 下午10:51, Arnd Bergmann 写道:
>>> On Fri, Aug 6, 2021 at 5:01 AM Xianting Tian
>>>> +#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))
>>> I think you need a higher alignment for DMA buffers, instead of sizeof(long),
>>> I would suggest ARCH_DMA_MINALIGN.
>> As some ARCH(eg, x86, riscv) doesn't define ARCH_DMA_MINALIG, so i think
>> it 's better remain the code unchanged,
>>
>> I will send v5 patch soon.
> I think you could just use "L1_CACHE_BYTES" as the alignment in this case.
> This will make the structure slightly larger for architectures that do not have
> alignment constraints on DMA buffers, but using a smaller alignment is
> clearly wrong. Another option would be to use ARCH_KMALLOC_MINALIGN.
yes, I unstand you, the align size must  L1_CACHE_BYTES at least.
>
> Note that there is a patch to add ARCH_DMA_MINALIGN to riscv already,
yes, I summited this patch, it is discussing, seems they don't want to 
apply it.
> as some implementations do not have coherent DMA. I had failed to
> realized though that on x86 you do not get an ARCH_DMA_MINALIGN
> definition.
I didn't find the definition in arch/x86/include/asm/cache.h and other 
place, x86 is dma coherent, it may doesn't need it.
>
>         Arnd