Re: [PATCH v4 1/4] dt-bindings: i2c: aspeed: add transfer mode support

[Brendan Higgins <brendanhiggins@google.com>]

On Thu, Apr 8, 2021 at 10:50 AM Jae Hyun Yoo
<jae.hyun.yoo@linux.intel.com> wrote:
>
> Ping.
>
> On 3/10/2021 7:55 AM, Jae Hyun Yoo wrote:
> > On 3/9/2021 6:15 PM, Rob Herring wrote:
> >> On Tue, Mar 9, 2021 at 10:02 AM Jae Hyun Yoo
> >> <jae.hyun.yoo@linux.intel.com> wrote:
> >>>
> >>> Hi Rob,
> >>>
> >>> On 3/6/2021 12:30 PM, Rob Herring wrote:
> >>>> On Wed, Feb 24, 2021 at 11:17:17AM -0800, Jae Hyun Yoo wrote:
> >>>>> Append bindings to support transfer mode.
> >>>>>
> >>>>> Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
> >>>>> Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
> >>>>> ---
> >>>>> Changes since v3:
> >>>>> - None
> >>>>>
> >>>>> Changes since v2:
> >>>>> - Moved SRAM resources back to default dtsi and added mode selection
> >>>>>     property.
> >>>>>
> >>>>> Changes since v1:
> >>>>> - Removed buffer reg settings from default device tree and added
> >>>>> the settings
> >>>>>     into here to show the predefined buffer range per each bus.
> >>>>>
> >>>>>    .../devicetree/bindings/i2c/i2c-aspeed.txt    | 37
> >>>>> +++++++++++++++----
> >>>>>    1 file changed, 30 insertions(+), 7 deletions(-)
> >>>>>
> >>>>> diff --git a/Documentation/devicetree/bindings/i2c/i2c-aspeed.txt
> >>>>> b/Documentation/devicetree/bindings/i2c/i2c-aspeed.txt
> >>>>> index b47f6ccb196a..242343177324 100644
> >>>>> --- a/Documentation/devicetree/bindings/i2c/i2c-aspeed.txt
> >>>>> +++ b/Documentation/devicetree/bindings/i2c/i2c-aspeed.txt
> >>>>> @@ -17,6 +17,20 @@ Optional Properties:
> >>>>>    - bus-frequency    : frequency of the bus clock in Hz defaults
> >>>>> to 100 kHz when not
> >>>>>                 specified
> >>>>>    - multi-master     : states that there is another master active
> >>>>> on this bus.
> >>>>> +- aspeed,i2c-xfer-mode      : should be "byte", "buf" or "dma" to
> >>>>> select transfer
> >>>>> +                      mode defaults to "byte" mode when not
> >>>>> specified.
> >>>>> +
> >>>>> +                      I2C DMA mode on AST2500 has these restrictions:
> >>>>> +                        - If one of these controllers is enabled
> >>>>> +                            * UHCI host controller
> >>>>> +                            * MCTP controller
> >>>>> +                          I2C has to use buffer mode or byte mode
> >>>>> instead
> >>>>> +                          since these controllers run only in DMA
> >>>>> mode and
> >>>>> +                          I2C is sharing the same DMA H/W with them.
> >>>>> +                        - If one of these controllers uses DMA
> >>>>> mode, I2C
> >>>>> +                          can't use DMA mode
> >>>>> +                            * SD/eMMC
> >>>>> +                            * Port80 snoop
> >>>>
> >>>> How does one decide between byte or buf mode?
> >>>
> >>> If a given system makes just one byte r/w transactions most of the time
> >>> then byte mode will be a right setting. Otherwise, buf mode is more
> >>> efficient because it doesn't generate a bunch of interrupts on every
> >>> byte handling.
> >>
> >> Then why doesn't the driver do byte transactions when it gets small
> >> 1-4? byte transactions and buffer transactions when it gets larger
> >> sized transactions.
> >
> > Good question and it could be an option of this implementation.
> > Actually, each mode needs different register handling so we need to add
> > additional conditional branches to make it dynamic mode change depends
> > on the data size which can be a downside. Also, checked that small
> > amount of data transfer efficiency in 'buf' transfer mode is almost same
> > to 'byte' mode so there would be no big benefit from the dynamic mode
> > change. Of course, we can remove the 'byte' transfer mode but we should
> > also provide flexibility of configuration on what this hardware can
> > support, IMO.

I would rather set the choice in device tree or Kconfig, which the
former is what I think you did here.

As for doing byte mode for small transactions and buffer/DMA for large
transactions, I would prefer sticking to a single mode based on what
is selected at build/boot time. Seems less error prone to me. Then
again, Rob probably has more experience in this area than I do, so
maybe this kind of thing is pretty common and I just don't realize it.

In any case, as for getting rid of byte mode, I would support that,
but not in this patch set. I would rather switch the default and get
users on buffer/DMA mode before taking away a fallback option.

My 2 cents, but I think the OzLabs and other active OpenBMC people are
probably a little more up to date on this.

Cheers