netdev - Re: [RFC PATCH 1/3] can: m_can: Create m_can core to leverage common code

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Date:   Thu, 10 Jan 2019 08:44:13 +0100
From:   Wolfgang Grandegger <wg@...ndegger.com>
To:     Dan Murphy <dmurphy@...com>, mkl@...gutronix.de,
        davem@...emloft.net
Cc:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] can: m_can: Create m_can core to leverage common
 code

Hello Dan,

sorry for my late response on that topic...

Am 09.01.19 um 21:58 schrieb Dan Murphy:
> Wolfgang
> 
> On 11/3/18 5:45 AM, Wolfgang Grandegger wrote:
>> Hello Dan,
>>
>> Am 31.10.2018 um 21:15 schrieb Dan Murphy:
>>> Wolfgang
>>>
>>> Thanks for the review
>>>
>>> On 10/27/2018 09:19 AM, Wolfgang Grandegger wrote:
>>>> Hello Dan,
>>>>
>>>> for the RFC, could you please just do the necessary changes to the
>>>> existing code. We can discuss about better names, etc. later. For
>>>> the review if the common code I quickly did:
>>>>
>>>>   mv m_can.c m_can_platform.c
>>>>   mv m_can_core.c m_can.c
>>>>
>>>> The file names are similar to what we have for the C_CAN driver.
>>>>
>>>>   s/classdev/priv/
>>>>   variable name s/m_can_dev/priv/
>>>>
>>>> Then your patch 1/3 looks as shown below. I'm going to comment on that
>>>> one. The comments start with "***"....
>>>>
>>>
>>> So you would like me to align the names with the c_can driver?
>>
>> That would be the obvious choice.
>>> <snip>
>>>>
>>>> *** I didn't review the rest of the patch for now.
>>>>
>>>
>>> snipped the code to reply to the comment.
>>>
>>>> Looking to the generic code, you didn't really change the way
>>>> the driver is accessing the registers. Also the interrupt handling
>>>> and rx polling is as it was before. Does that work properly using
>>>> the SPI interface of the TCAN4x5x?
>>>
>>> I don't want to change any of that yet.  Maybe my cover letter was not clear
>>> or did not go through.
>>>
>>> But the intention was just to break out the functionality to create a MCAN framework
>>> that can be used by devices that contain the Bosch MCAN core and provider their own protocal to access
>>> the registers in the device.
>>>
>>> I don't want to do any functional changes at this time on the IP code itself until we have a framework.
>>> There should be no regression in the io mapped code.
>>>
>>> I did comment on the interrupt handling and asked if a threaded work queue would affect CAN timing.
>>> For the original TCAN driver this was the way it was implemented.
>>
>> Do threaded interrupts with RX polling make sense? I think we need a
>> common interface allowing to select hard-irqs+napi or threaded-irqs.
>>
> 
> I have been working on this code for about a month now and I am *not happy* with the amount of change that needs
> to be done to make the m_can a framework.
> 
> I can tx/rx frames from another CAN device to the TCAN part but I have not even touched the iomapped code.
> 
> The challenging part is that the m_can code that is currently available does not have to worry about atomic context because
> there is no peripheral waiting.  Since the TCAN is a peripheral device we need to take into about the hard waits in IRQ context
> as well as the atomic context.  Doing this creates many deltas in the base code that may break iomapped devices.  I have had to 
> add the thread_irqs and now I am in the midst of the issue you brought up with napi.  I would have to schedule a queue for perp devices
> and leave the non-threaded iomapped irq.
> 
> At this point I think it may be wise to leave the m_can code alone as it is working and stable and just work on the TCAN driver as
> a standalone driver.  A framework would be nice but I think it would destablize the m_can driver which is embedded in many SoC's and
> we cannot possibly test everyone of them.

Unfortunately, I do not have m_can hardware at hand.

> What are your thoughts?

What we need is a common set of functions doing tx, rx, error and state
handling. This will requires substantial changes to the existing
io-mapped m_can driver, of course. I still believe it's worth the
effort, but I agree that it's difficult for you to re-write and test the
existing m_can driver.

What about implementing such a set of common functions plus the SPI
specific part for your TCAN device. What do you/others think?

Wolfgang.