linux-kernel - Re: [PATCH v7 4/5] spi: dw: Add DMA address widths capability check

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Message-ID: <ZD/VO1cuBYGCP4O2@smile.fi.intel.com>
Date:   Wed, 19 Apr 2023 14:49:15 +0300
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Joy Chakraborty <joychakr@...gle.com>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org, manugautam@...gle.com,
        rohitner@...gle.com
Subject: Re: [PATCH v7 4/5] spi: dw: Add DMA address widths capability check

On Wed, Apr 19, 2023 at 11:18:25AM +0530, Joy Chakraborty wrote:
> On Tue, Apr 18, 2023 at 1:08 PM Andy Shevchenko
> <andriy.shevchenko@...el.com> wrote:
> > On Tue, Apr 18, 2023 at 05:29:01AM +0000, Joy Chakraborty wrote:

...

> > > +     /*
> > > +      * Assuming both channels belong to the same DMA controller hence 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 hence
>          * the peripheral side address width capabilities most likely would
>          * 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?

> 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 capabilities.
> 
> Please let me know if you think otherwise.

-- 
With Best Regards,
Andy Shevchenko