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    SubjectRe: [PATCH] i2c: i2c-qcom-geni: Properly handle DMA safe buffers
    Date
    Quoting Wolfram Sang (2018-09-18 15:16:46)
    >
    > > This fixes a problem where the kernel oopses cleaning pages for a buffer
    > > that's mapped into the vmalloc space. The pages are returned from
    > > request_firmware() and passed down directly to the i2c master to write
    > > to the i2c touchscreen device. Mapping vmalloc buffers with
    > > dma_map_single() won't work reliably, causing an oops like below:
    >
    > Exactly the reason why I implemented I2C_M_DMA_SAFE. Did you also notice
    > the helper i2c_get_dma_safe_msg_buf() which you maybe could use for
    > len > 32?
    >

    Yes I noticed that after sending the patch, thanks for pointing it out.

    But now when I try to use it I'm not exicted when the buffer is bounced
    but we fail to map the buffer with the DMA APIs. For an I2C_M_RD
    message, presumably we would call i2c_release_dma_safe_msg_buf() in this
    error case, but that will cause the original buffer to be copied over
    which seems wasteful to do, but I guess it's OK. I suppose we could have
    another function like:

    void i2c_release_dma_safe_msg_buf_on_err(struct i2c_msg *msg, u8 *buf)
    {
    if (!buf || buf == msg->buf)
    return;

    kfree(buf);
    }

    so that we don't copy over the buffer on failure and still properly free
    the buffer that we setup. Or we can pass an argument to
    i2c_release_dma_safe_msg_buf() to indicate if we should do the memcpy or
    not? Removing the I2C_M_RD flag from the message on failure doesn't
    sound like a good idea.

    Either way, I can resend the patch with the releasing and duplicate
    memcpy and we can discuss this minor optimization.

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