Date:   Mon, 20 Mar 2023 17:04:24 +0300
From:   Serge Semin <fancer.lancer@gmail.com>
To:     Joy Chakraborty <joychakr@google.com>
Cc:     Mark Brown <broonie@kernel.org>, linux-spi@vger.kernel.org,
        linux-kernel@vger.kernel.org, manugautam@google.com,
        rohitner@google.com
Subject: Re: [PATCH] spi: dw: Add 32 bpw support to DW DMA Controller
Message-ID: <20230320140424.pfy4zf6dfwgf2xyj@mobilestation>
References: <20230320055746.2070049-1-joychakr@google.com>
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In-Reply-To: <20230320055746.2070049-1-joychakr@google.com>
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Hello Joy.

On Mon, Mar 20, 2023 at 05:57:46AM +0000, Joy Chakraborty wrote:
> If DW Controller is capable of 32 bits per word support then SW or DMA
> controller has to write 32bit or 4byte data to the FIFO at a time.
> 
> This Patch adds support for AxSize = 4 bytes configuration from dw dma
> driver if n_bytes i.e. number of bytes per write to fifo is 4.
> 
> Signed-off-by: Joy Chakraborty <joychakr@google.com>

> ---
>  drivers/spi/spi-dw-dma.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/spi/spi-dw-dma.c b/drivers/spi/spi-dw-dma.c
> index ababb910b391..7d06ecfdebe1 100644
> --- a/drivers/spi/spi-dw-dma.c
> +++ b/drivers/spi/spi-dw-dma.c
> @@ -212,6 +212,8 @@ static enum dma_slave_buswidth dw_spi_dma_convert_width(u8 n_bytes)
>  		return DMA_SLAVE_BUSWIDTH_1_BYTE;
>  	else if (n_bytes == 2)
>  		return DMA_SLAVE_BUSWIDTH_2_BYTES;

> +	else if (n_bytes == 4)
> +		return DMA_SLAVE_BUSWIDTH_4_BYTES;

In case of the DFS-width being of 32-bits size n_bytes can be 4 and
theoretically _3_ (practically it's unluckily, but anyway). Here
it is:
...
if (dws->caps & DW_SPI_CAP_DFS32)
	master->bits_per_word_mask = SPI_BPW_RANGE_MASK(4, 32);
...
dws->n_bytes = DIV_ROUND_UP(transfer->bits_per_word, BITS_PER_BYTE);
...

So what about converting the dw_spi_dma_convert_width() method to
having the switch-case statement and adding the adjacent "case 3:
case 4:" statement there?

* We could add the individual case-3 branch with DMA_SLAVE_BUSWIDTH_3_BYTES
* returned, but the DMA-engines with 3-bytes bus width capability are
* so rare. So is the case of having n_bytes == 3. Thus I guess it
* won't hurt to extend the bus up to four bytes even though there are
* only three bytes required.

Please also note. Currently the spi-dw-dma.o driver doesn't make sure
that the requested buswidth is actually supported by the DMA-engine
(see dma_slave_caps.{src,dst}_addr_widths fields semantics). It would
be nice to have some sanity check in there, but until then note DMA
may still fail even if you specify a correct buswidth.

-Serge(y)

>  
>  	return DMA_SLAVE_BUSWIDTH_UNDEFINED;
>  }
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog
>