linux-kernel - Re: [PATCH v5 1/2] spi: dw: Add 32 bpw support to DW DMA Controller

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Date:   Tue, 11 Apr 2023 20:48:52 +0530
From:   Joy Chakraborty <joychakr@...gle.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.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 v5 1/2] spi: dw: Add 32 bpw support to DW DMA Controller

On Tue, Apr 11, 2023 at 8:47 PM Andy Shevchenko
<andriy.shevchenko@...el.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.

>
> > > > > > +     case 4:
> > > > > > +             return DMA_SLAVE_BUSWIDTH_4_BYTES;
> > > > > > +     default:
> > > > > > +             return DMA_SLAVE_BUSWIDTH_UNDEFINED;
> > > > > > +     }
>
> --
> With Best Regards,
> Andy Shevchenko
>
>
Thanks
Joy