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To:     sumit.semwal@linaro.org, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
From:   Alexey Skidanov <alexey.skidanov@intel.com>
Subject: dma-buf CPU access
Cc:     Laura Abbott <labbott@redhat.com>
Message-ID: <e2571723-1b92-db19-8ca8-06f358363453@intel.com>
Date:   Fri, 9 Feb 2018 23:25:44 +0200
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Hi,

According to the CPU access instructions in the dma-buf.c, I understand
that on 32 bit platforms, due to the limited number of persistent kernel
mappings or limited vmalloc range, the CPU access pattern should look
like this:

dma_buf_start_cpu_access();

void *ptr = dma_buf_kmap();

/*Access the page pointed by ptr*/

dma_buf_kunmap(ptr);

dma_buf_end_cpu_access();


or


dma_buf_start_cpu_access();

void *ptr = dma_buf_vmap();

/*Access the buffer pointed by ptr*/

dma_buf_vunmap(ptr);

dma_buf_end_cpu_access();


But on 64 bit platforms, there are no such restrictions (there is no
highmem at all). So, frequently accessed buffer, may be mapped once, but
every access should be surrounded by :

dma_buf_start_cpu_access();

/*Access the buffer pointed by ptr*/

dma_buf_end_cpu_access()

Am I correct?

Thanks,
Alexey