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From:   Joy Chakraborty <joychakr@google.com>
Date:   Wed, 19 Apr 2023 18:18:04 +0530
Message-ID: <CAOSNQF1wf3m+YTmh5qQWCM6+x3j2whvG6F=dW6Hd7zW0Y+E_1g@mail.gmail.com>
Subject: Re: [PATCH v7 4/5] spi: dw: Add DMA address widths capability check
To:     Andy Shevchenko <andriy.shevchenko@intel.com>
Cc:     Serge Semin <fancer.lancer@gmail.com>,
        Mark Brown <broonie@kernel.org>, linux-spi@vger.kernel.org,
        linux-kernel@vger.kernel.org, manugautam@google.com,
        rohitner@google.com
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On Wed, Apr 19, 2023 at 5:19=E2=80=AFPM Andy Shevchenko
<andriy.shevchenko@intel.com> wrote:
>
> On Wed, Apr 19, 2023 at 11:18:25AM +0530, Joy Chakraborty wrote:
> > On Tue, Apr 18, 2023 at 1:08=E2=80=AFPM Andy Shevchenko
> > <andriy.shevchenko@intel.com> wrote:
> > > On Tue, Apr 18, 2023 at 05:29:01AM +0000, Joy Chakraborty wrote:
>
> ...
>
> > > > +     /*
> > > > +      * Assuming both channels belong to the same DMA controller h=
ence the
> > > > +      * address width capabilities most likely would be the same.
> > > > +      */
> > >
> > > I had a small comment on this In v6 thread.
> >
> > Sure,
> >
> > Your comment in V6 thread:
> > "
> > I would add something to explain the side of these address width, like
> >
> >          * Assuming both channels belong to the same DMA controller hen=
ce
> >          * the peripheral side address width capabilities most likely w=
ould
> >          * be the same.
> > "
> >
> > I do not think the address width capabilities are dependent on the
> > side of generation like memory or peripheral.
>
> Yes, they are independent. Memory could do with 4 bytes, while peripheral=
 with
> 1 byte and so on.
>
> > From what I understand,
> > address width capabilities are solely dependent on the transaction
> > generation capability of the DMA controller towards the system bus.
>
> What do you mean by a SB in the above? Memory? Peripheral?

By system bus I mean anything that is connecting the Memory, DMA and
the peripheral.
Something like :

          +-----------+          +-------------------+
          |               |           |                        |
          |   DMA    |           | PERIPHERAL |
          |               |           |                         |
          +----^-+---+          +-----+--^---------+
        *** -->| |                         |    |
                  | |                         |    |
<------------+-v--------------------v---+------------->
                    SYSTEM BUS
<---------------------+--^----------------------------->
                            |   |
                            |   |
                     +----v--+-----+
                     |                   |
                     |  MEMORY |
                     |                   |
                     +--------------+
*** : Address width capabilities should be the capability of the DMA
to generate transactions to the system bus on the marked interface
irrespective of whether it is destined for Peripheral or memory is
what I understand.

>
> > What we intend to highlight here is the assumption that both tx and rx
> > channel would belong to the same DMA controller hence the transaction
> > generation capabilities would be the same both for read and write
> > (e.g. if the DMA controller is able to generate 32 bit sized reads
> > then it should also be able to generate 32 bit sized writes).
> > With this assumption we are doing a bitwise and of both tx and rx capab=
ilities.
> >
> > Please let me know if you think otherwise.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>