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Date:   Tue, 11 Apr 2023 19:21:57 +0300
From:   Andy Shevchenko <andriy.shevchenko@intel.com>
To:     Joy Chakraborty <joychakr@google.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
Subject: Re: [PATCH v5 1/2] spi: dw: Add 32 bpw support to DW DMA Controller
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On Tue, Apr 11, 2023 at 08:48:52PM +0530, Joy Chakraborty wrote:
> On Tue, Apr 11, 2023 at 8:47 PM Andy Shevchenko
> <andriy.shevchenko@intel.com> wrote:
> > On Tue, Apr 11, 2023 at 06:09:16PM +0300, Serge Semin wrote:
> > > On Tue, Apr 11, 2023 at 05:46:34PM +0300, Andy Shevchenko wrote:
> > > > On Tue, Apr 11, 2023 at 05:11:15PM +0300, Serge Semin wrote:
> > > > > On Tue, Apr 11, 2023 at 03:13:49PM +0300, Andy Shevchenko wrote:
> > > > > > On Thu, Mar 30, 2023 at 06:34:49AM +0000, Joy Chakraborty wrote:

...

> > > > > > > -     if (n_bytes == 1)
> > > > > > > +     switch (n_bytes) {
> > > > > > > +     case 1:
> > > > > > >               return DMA_SLAVE_BUSWIDTH_1_BYTE;
> > > > > > > -     else if (n_bytes == 2)
> > > > > > > +     case 2:
> > > > > > >               return DMA_SLAVE_BUSWIDTH_2_BYTES;
> > > > > > > -
> > > > > > > -     return DMA_SLAVE_BUSWIDTH_UNDEFINED;
> > > > > >
> > > > > > > +     case 3:
> > > > > >
> > > > > > I'm not sure about this.
> > > > >
> > > > > This actually makes sense seeing the function argument can have values
> > > > > 1, 2, _3_ and 4:
> > > > > dws->n_bytes = DIV_ROUND_UP(transfer->bits_per_word, BITS_PER_BYTE);
> > > > > transfer->bits_per_word = __F__(master->bits_per_word_mask = SPI_BPW_RANGE_MASK(4, 32));
> > > > > ...
> > > > > dw_spi_dma_convert_width(dws->n_bytes)
> > > > >
> > > > > The spi_transfer.bits_per_word field value depends on the
> > > > > SPI peripheral device communication protocol requirements which may
> > > > > imply the 3-bytes word xfers (even though it's indeed unluckily).
> > > > >
> > > > > This semantic will also match to what we currently have in the
> > > > > IRQ-based SPI-transfer implementation (see dw_writer() and
> > > > > dw_reader()).
> > >
> > > > Nice, but we have DMA_SLAVE_BUSWIDTH_3_BYTES definition for that. Why we don't
> > > > use it?
> > >
> > > We could but there are two more-or-less firm reasons not to do
> > > that:
> > > 1. There aren't that much DMA-engines with the
> > > DMA_SLAVE_BUSWIDTH_3_BYTES capability meanwhile the DW APB SSI just
> > > ignores the upper bits if CTRLR0.DFS is less than the value actual
> > > written to the DR registers. Note DW DMAC engine isn't one of such
> > > controllers. So if we get to meet a peripheral SPI-device with 3-bytes
> > > word protocol transfers and the DMA-engine doesn't support it the
> > > DMA-based transfers may fail (depending on the DMA-engine driver
> > > implementation).
> > > 2. The DW APB SSIs (3.x and 4.x) can be synthesized with the APB Data
> > > Bus Width of 8, 16 and 32. So no matter whether DMA-engine supports
> > > the 3-bytes bus width the system bus most likely will either convert
> > > the transfers to the proper sized bus-transactions or fail.
> > >
> > > So taking all of the above into account not using the
> > > DMA_SLAVE_BUSWIDTH_3_BYTES macro here seems better than using it with
> > > a risk to fail some of the platform setups especially seeing the DW
> > > APB SSI ignores the upper bits anyway.
> >
> > But this is not about SPI host hardware, it's about the consumers.
> > They should know about supported sizes. Either we add the corresponding support
> > to the driver or remove 3 case as I suggested. I don't think it's correct to
> > use 3 as 4.
> 
> Another thing to add is that as per spi.h even if bits per word is a
> not a power of 2 the buffer should be formatted in memory as a power
> of 2
> ...
>  * @bits_per_word: Data transfers involve one or more words; word sizes
>  * like eight or 12 bits are common.  In-memory wordsizes are
>  * powers of two bytes (e.g. 20 bit samples use 32 bits).
>  * This may be changed by the device's driver, or left at the
>  * default (0) indicating protocol words are eight bit bytes.
>  * The spi_transfer.bits_per_word can override this for each transfer.
> ...
> Hence for n_bytes = 3 or 24 bits/per word we expect the client SW to
> format it to 4 byte buffers hence the transaction generated should
> also be of size 4 from the DMA.

You didn't get my point. The consumer wants to know if the 3 bytes is supported
or not, that's should be part of the DMA related thing, right?

It's incorrectly to say 4 for 3 if the backend DMA controller behaves differently
on this. How do you know that (any) DMA controller integrated with this hardware
has no side effects for this change.

So, I don't think the case 3 is correct in this patch.

> > > > > > > +     case 4:
> > > > > > > +             return DMA_SLAVE_BUSWIDTH_4_BYTES;
> > > > > > > +     default:
> > > > > > > +             return DMA_SLAVE_BUSWIDTH_UNDEFINED;
> > > > > > > +     }

-- 
With Best Regards,
Andy Shevchenko