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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 18 Feb 2020 15:34:06 +0100
Message-ID: <CAK8P3a2gxgsJskSVVGLBRkYTqBbJs20Mwe-x1FExiUeNBN9eUQ@mail.gmail.com>
Subject: Re: [PATCH v2 11/16] bus: mhi: core: Add support for data transfer
To:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc:     gregkh <gregkh@linuxfoundation.org>, smohanad@codeaurora.org,
        Jeffrey Hugo <jhugo@codeaurora.org>,
        Kalle Valo <kvalo@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        hemantk@codeaurora.org,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Tue, Feb 18, 2020 at 6:51 AM Manivannan Sadhasivam
<manivannan.sadhasivam@linaro.org> wrote:
> On Mon, Feb 17, 2020 at 10:17:51PM +0530, Manivannan Sadhasivam wrote:
> > On Mon, Feb 17, 2020 at 05:13:37PM +0100, Arnd Bergmann wrote:
> > > On Fri, Jan 31, 2020 at 2:51 PM Manivannan Sadhasivam
> > > <manivannan.sadhasivam@linaro.org> wrote:
> > > While looking through the driver to see how the DMA gets handled, I came
> > > across the multitude of mhi_queue_* functions, which seems like a
> > > layering violation to me, given that they are all implemented by the
> > > core code as well, and the client driver needs to be aware of
> > > which one to call. Are you able to lift these out of the common interface
> > > and make the client driver call these directly, or maybe provide a direct
> > > interface based on mhi_buf_info to replace these?
> > >
> >
> > It sounds reasonable to me. Let me discuss this internally with Qcom guys to
> > see if they have any objections.
> >
>
> I looked into this in detail and found that the queue_xfer callbacks are tied
> to the MHI channels. For instance, the callback gets attached to ul_chan (uplink
> channel) and dl_chan (downlink channel) during controller registration. And
> when the device driver calls the callback, the MHI stack calls respective queue
> function for the relevant channel. For instance,
>
> ```
> int mhi_queue_transfer(struct mhi_device *mhi_dev,
>                        enum dma_data_direction dir, void *buf, size_t len,
>                        enum mhi_flags mflags)
> {
>         if (dir == DMA_TO_DEVICE)
>                 return mhi_dev->ul_chan->queue_xfer(mhi_dev, mhi_dev->ul_chan,
>                                                     buf, len, mflags);
>         else
>                 return mhi_dev->dl_chan->queue_xfer(mhi_dev, mhi_dev->dl_chan,
>                                                     buf, len, mflags);
> }
> ```
>
> If we use the direct queue API's this will become hard to handle. So, I'll keep
> it as it is.

Please have another look, this is exactly the part of the subsystem
that I think should be replaced. For the caller, there should not be much
difference between passing ul_chan/dl_chan or
DMA_TO_DEVICE/DMA_FROM_DEVICE, so that too can be lifted
to a higher level.

      Arnd