Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

From: Jonathan Richardson
Date: Thu May 14 2015 - 14:55:53 EST

On 15-05-14 11:36 AM, Scott Branden wrote:
> On 15-05-14 11:28 AM, Florian Fainelli wrote:
>> On 14/05/15 11:19, Scott Branden wrote:
>>> Hi Mark,
>>> On 15-05-14 03:31 AM, Mark Brown wrote:
>>>> On Wed, May 13, 2015 at 05:19:06PM -0700, Scott Branden wrote:
>>>>> The purpose of this mspi interface is to connect to NOR flash. There
>>>>> are
>>>>> other SPI interfaces on the devices used to connect to other SPI
>>>>> devies. We
>>>>> don't have any need to support full duplex slaves on this port (NOR
>>>>> have any
>>>>> hardware wired this way - pun realized after typing this).
>>>> Chip vendors often say this sort of thing and then get surprised by
>>>> what
>>>> their users choose to do, and even if it only ever gets used with flash
>>>> all it would take is some new flash command which can use full duplex
>>>> for something. Please write the code so it at least tries to handle
>>>> full duplex operation, if you can't test it fully that's not the end of
>>>> the world. It doesn't look like it should be particularly difficult.
>>> Yes, there is always room for improvements in code. In this case - it
>>> really is not worth our time to add code we can't test. We try to
>>> deliver code that we can test and actually works. Yes, if anyone needs
>>> to use the mspi for full duplex operation code can be added in the
>>> future - it is software. This block has gone through many generations
>>> of our SoCs and has only been used for this purpose - the bootROM boots
>>> from this SPI only. It is dedicated for this purpose.
>>> Also, there are other SPI blocks on the SoC that are used for connect
>>> other SPI devices. These SPI blocks are properly designed for
>>> full-duplex operation. This mspi block is designed for connecting to
>>> NOR flash.
>> This is an implementation detail and architectural choice that is
>> specific to the Cygnus SoCs where a single MSPI is present, right?
>> This same MSPI block is instantiated multiple times in BCM7xxx chips to
>> interface to general purpose SPI slaves, ranging from Ethernet switches
>> to bluetooth chips etc... those will definitively want full-duplex to be
>> working.
>> I would greatly appreciate if some thought to supporting full duplex
>> transfers was given in this driver drop. If you cannot test that, then
>> maybe just flag the driver has been half-duplex only for now...
>> Thank you.
> Hi Florian,
> Yes, we can flag that - but we have no understanding of how the block
> operates in this mode. It would be great if your team could add the
> necessary full duplex support in a patch enhancement to the driver?

Going blind on this one isn't the way to go. If I had a real full duplex
slave I could experiment with it to get it working. But with the docs I
have on this and considering how finicky the block is I think we're
wasting our time.


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