linux-kernel - Re: [PATCH v2 02/16] bus: mhi: core: Add support for registering MHI controllers

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Message-ID: <20200217052743.GA4809@Mani-XPS-13-9360>
Date:   Mon, 17 Feb 2020 10:57:43 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.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

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);
```

There was some discussion about it here: https://lkml.org/lkml/2020/1/27/21

The MHI bus itself has the struct device and it is the child of the physical
bus (PCI-E in this case).

Now coming to your actual question of why using a custom "MHI" prefix for
dev_ APIs. I agree that if we use the struct device of MHI bus it is not at all
needed. The fact that we are using "mhi_cntrl->dev" (which points to PCI-E dev)
is what confusing and it can be avoided.

I'll change it in next iteration.

Thanks,
Mani

> thanks,
> 
> greg k-h