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Date:   Mon, 17 Feb 2020 12:59:30 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     arnd@...db.de, smohanad@...eaurora.org, jhugo@...eaurora.org,
        kvalo@...eaurora.org, bjorn.andersson@...aro.org,
        hemantk@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/16] bus: mhi: core: Add support for registering MHI
 controllers

On Mon, Feb 17, 2020 at 10:57:43AM +0530, Manivannan Sadhasivam wrote:
> Hi Greg,
> 
> On Thu, Feb 13, 2020 at 07:53:02AM -0800, Greg KH wrote:
> > On Thu, Feb 13, 2020 at 09:18:09PM +0530, Manivannan Sadhasivam wrote:
> > > Hi Greg,
> > > 
> > > On Thu, Feb 13, 2020 at 07:34:18AM -0800, Greg KH wrote:
> > > > On Thu, Feb 13, 2020 at 08:50:13PM +0530, Manivannan Sadhasivam wrote:
> > > > > On Tue, Feb 11, 2020 at 11:20:55AM -0800, Greg KH wrote:
> > > > > > On Wed, Feb 12, 2020 at 12:11:30AM +0530, Manivannan Sadhasivam wrote:
> > > > > > > Hi Greg,
> > > > > > > 
> > > > > > > On Thu, Feb 06, 2020 at 05:57:55PM +0100, Greg KH wrote:
> > > > > > > > On Fri, Jan 31, 2020 at 07:19:55PM +0530, Manivannan Sadhasivam wrote:
> > > > > > > > > --- /dev/null
> > > > > > > > > +++ b/drivers/bus/mhi/core/init.c
> > > > > > > > > @@ -0,0 +1,407 @@
> > > > > > > > > +// SPDX-License-Identifier: GPL-2.0
> > > > > > > > > +/*
> > > > > > > > > + * Copyright (c) 2018-2020, The Linux Foundation. All rights reserved.
> > > > > > > > > + *
> > > > > > > > > + */
> > > > > > > > > +
> > > > > > > > > +#define dev_fmt(fmt) "MHI: " fmt
> > > > > > > > 
> > > > > > > > This should not be needed, right?  The bus/device name should give you
> > > > > > > > all you need here from what I can tell.  So why is this needed?
> > > > > > > > 
> > > > > > > 
> > > > > > > The log will have only the device name as like PCI-E. But that won't specify
> > > > > > > where the error is coming from. Having "MHI" prefix helps the users to
> > > > > > > quickly identify that the error is coming from MHI stack.
> > > > > > 
> > > > > > If the driver binds properly to the device, the name of the driver will
> > > > > > be there in the message, so I suggest using that please.
> > > > > > 
> > > > > > No need for this prefix...
> > > > > > 
> > > > > 
> > > > > So the driver name will be in the log but that won't help identifying where
> > > > > the log is coming from. This is more important for MHI since it reuses the
> > > > > `struct device` of the transport device like PCI-E. For instance, below is
> > > > > the log without MHI prefix:
> > > > > 
> > > > > [   47.355582] ath11k_pci 0000:01:00.0: Requested to power on
> > > > > [   47.355724] ath11k_pci 0000:01:00.0: Power on setup success
> > > > > 
> > > > > As you can see, this gives the assumption that the log is coming from the
> > > > > ath11k_pci driver. But the reality is, it is coming from MHI bus.
> > > > 
> > > > Then you should NOT be trying to "reuse" a struct device.
> > > > 
> > > > > With the prefix added, we will get below:
> > > > > 
> > > > > [   47.355582] ath11k_pci 0000:01:00.0: MHI: Requested to power on
> > > > > [   47.355724] ath11k_pci 0000:01:00.0: MHI: Power on setup success
> > > > > 
> > > > > IMO, the prefix will give users a clear idea of logs and that will be very
> > > > > useful for debugging.
> > > > > 
> > > > > Hope this clarifies.
> > > > 
> > > > Don't try to reuse struct devices, if you are a bus, have your own
> > > > devices as that's the correct way to do things.
> > > > 
> > > 
> > > I assumed that the buses relying on a different physical interface for the
> > > actual communication can reuse the `struct device`. I can see that the MOXTET
> > > bus driver already doing it. It reuses the `struct device` of SPI.
> > 
> > How can you reuse anything?
> > 
> > > And this assumption has deep rooted in MHI bus design.
> > 
> > Maybe I do not understand what this is at all, but a device can only be
> > on one "bus" at a time.  How is that being broken here?
> > 
> 
> Let me share some insight on how it is being used:
> 
> The MHI bus sits on top of the actual physical bus like PCI-E and requires
> the physical bus for doing activities like allocating I/O virtual address,
> runtime PM etc... The part which gets tied to the PCI-E from MHI is called MHI
> controller driver. This MHI controller driver is also the actual PCI-E driver
> managing the device.
> 
> For instance, we have QCA6390 PCI-E WLAN device. For this device, there is a
> ath11k PCI-E driver and the same driver also registers as a MHI controller and
> acts as a MHI controller driver. This is where I referred to reusing the PCI-E
> struct device. It's not that MHI bus itself is reusing the PCI-E struct device
> but we need the PCI-E device pointer to do above mentioned IOVA, PM operations
> in some places. One of the usage is below:
> 
> ```
> void *buf = dma_alloc_coherent(mhi_cntrl->dev, size, dma_handle, gfp);
> ```

Wait, why do you need to call this with the parent dev?  Why not with
your struct device?  What does the parent pointer have that yours does
not?  Is it not correctly having whatever dma attributes the parent has
set properly for your device as well?  If not, why not just fix that and
then _your_ device can be doing the allocation?

thanks,

greg k-h

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