linux-kernel - Re: [PATCH v5 3/9] spi: support controllers with multiple data lanes

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Message-ID: <8e12d2a5-3780-45af-a70c-4c112184fcc6@baylibre.com>
Date: Fri, 16 Jan 2026 17:12:09 -0600
From: David Lechner <dlechner@...libre.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>,
 Mark Brown <broonie@...nel.org>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Marcelo Schmitt <marcelo.schmitt@...log.com>,
 Michael Hennerich <michael.hennerich@...log.com>,
 Nuno Sá <nuno.sa@...log.com>,
 Jonathan Cameron <jic23@...nel.org>, Andy Shevchenko <andy@...nel.org>,
 Sean Anderson <sean.anderson@...ux.dev>, linux-spi@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-iio@...r.kernel.org
Subject: Re: [PATCH v5 3/9] spi: support controllers with multiple data lanes

On 1/12/26 1:35 PM, Andy Shevchenko wrote:
> On Mon, Jan 12, 2026 at 07:11:26PM +0000, Mark Brown wrote:
>> On Mon, Jan 12, 2026 at 09:07:17PM +0200, Andy Shevchenko wrote:
>>> On Mon, Jan 12, 2026 at 11:45:21AM -0600, David Lechner wrote:
> 
> ...
> 
>>>> +	/* Multi-lane SPI controller support. */
>>>> +	u32			tx_lane_map[SPI_DEVICE_DATA_LANE_CNT_MAX];
>>>> +	u32			num_tx_lanes;
>>>> +	u32			rx_lane_map[SPI_DEVICE_DATA_LANE_CNT_MAX];
>>>> +	u32			num_rx_lanes;
>>
>>> This adds 72 bytes in _each_ instance of spi_device on the platforms that do
>>> not use the feature and might not ever use it. Can we move to the pointer
>>> and allocate the mentioned fields separately, please?
>>
>> Do we have real systems where we have enough SPI devices for anyone to
>> care?
> 
> Define "enough" :-) To me even dozen of devices is enough (it gets almost a 1kB
> of space) esp. if we are talking about quite low profile embedded systems.
> 

We could make it u8 and save the same amount (on 64-bit systems) while avoiding
the extra complexity of separate allocation.

I'm not particularly keen on requiring `/bits/ 8` in the devicetree though since
it is unusual and often trips people up.